Mercedes and Ferrari had impressive starts to pre-season testing as Red Bull and McLaren hit trouble.
Lewis Hamilton was fastest as Mercedes completed more than 140 laps with both their drivers at Circuit de Catalunya.
Hamilton was 0.113 seconds faster than Sebastian Vettel's Ferrari, which also ran reliably and used the slower medium tyre with the Mercedes on the soft.
By contrast, the mileage of Red Bull and McLaren was limited by recurring reliability problems in Barcelona.
Hamilton's team-mate Valtteri Bottas, a replacement for the now-retired world champion Nico Rosberg, was fifth fastest.
The Finn was 1.404secs slower than British three-time champion Hamilton but as he ran in the morning rather than the afternoon the times cannot realistically be compared.
Bottas said: "It's very nice to start proper testing and it was a good morning, lot of laps, no issues and some good tests."
F1 has introduced new rules this season aimed at making the cars up to five seconds a lap faster and more demanding for the drivers.
"It is a bigger car. It has more downforce and you can feel it is heavier. The corner speeds are quite a bit higher," added Bottas.
"It is more demanding but luckily I am well prepared for the season. I like the stability especially in the high-speed corners. There is a lot of grip in the car. I very much enjoyed it."
Not a good start for some
The first day of pre-season testing is all about ironing out problems and beginning to understand how the cars work.
As such, no team begins by trying to set the fastest possible lap times.
At this stage, mileage is key, which is why the truncated days suffered by Red Bull and McLaren are bad news with only eight days of running before the start of the season.
Alonso suffered an oil system problem after just a single installation lap, which cost him the whole morning session.
Honda changed the engine for the afternoon and the double world champion from Spain was able to get out on track for a few runs with two hours of the day remaining - but still managed less than a quarter of the laps achieved by Mercedes.
Honda has fundamentally revised its engine design for this season, effectively following the same route as Mercedes have used since the start of the turbo hybrid formula in 2014, and there are clearly still issues to resolve.
McLaren racing director Eric Boullier said Alonso was "not very happy" about the development.
Red Bull, who have hopes of challenging Mercedes this year, blamed problems with a sensor for Daniel Ricciardo managing only five laps in the morning.
He did a further three early in the afternoon before another lengthy visit to the pits because of a battery problem. He did finally get in some running and ended the day with just under 50 laps.
Ricciardo ended up with fifth fastest time and team boss Christian Horner said the problems were "not major issues".
Mercedes look formidable
In contrast to their rivals, Mercedes made a typically strong start to their preparations for the season.
New signing Bottas completed 79 laps - more than a grand prix distance - in the morning, ending up second fastest to Vettel at the time.
Hamilton took over in the afternoon, with Mercedes fitting a 'shark-fin' engine cover for the first time, and was quickly up to speed, completing more than 60 laps himself.
Ferrari and Williams also had good days, both completing more than 100 laps.
Vettel stuck to the 'medium' compound of tyre for most of the day before a brief run on the 'hard', so on the face of it his lap time looks impressive.
However, the German was also quickest for Ferrari on the first day of pre-season testing last season - and the Italian team ended the campaign winless for the second time in three years.
Felipe Massa, persuaded to come out of retirement to fill former team-mate Bottas' seat at Williams, was third fastest.
Barcelona pre-season test: day one results:
1 Lewis Hamilton (GB) Mercedes 1:21.765 *
2 Sebastian Vettel (Ger) Ferrari 1:21.878
3 Felipe Massa (Brz) Williams 1:22.076 *
4 Kevin Magnussen (Den) Haas 1:22.894 *
5 Daniel Ricciardo (Aus) Red Bull 1:22.926 *
6 Valtteri Bottas (Fin) Mercedes 1:23.169 *
7 Sergio Perez (Mex) Force India 1:23.709 *
8 Carlos Sainz (Spa) Toro Rosso 1:24.494
9 Nico Hulkenberg (Ger) Renault 1:24.784
10 Fernando Alonso (Spa) McLaren 1:24.852 *
11 Marcus Ericsson (Swe) Sauber 1:26.841
* time set on 'soft' tyres; all others set on medium tyres