World champion Lewis Hamilton signed off the first week of pre-season testing with an imposing fastest time.
The Mercedes driver was 0.5 seconds clear of the rest and 0.9 sec quicker than 2017 title rival, Ferrari's Sebastian Vettel, despite using a harder tyre.
Pre-season times are notoriously unreliable indicators of form because car specifications are not revealed and many factors can affect performance.
But Hamilton's pace underlines Mercedes' pre-season favourites status. "I was really happy to get out today," Hamilton said. "I don't normally like testing, but we hadn't had many miles so I was keen to stretch the legs on the new car and so far it is positive."
But he said the new track surface at Barcelona was making it difficult to work out how good the car was.
"The track is quicker than it was, much smoother, a lot more grip so it is difficult to tell whether the car is that much better, how much is track, tyres," he said.
"The next few days we'll get a better understanding. It is not the best of readings but it is positive. It definitely does feel like a better car so that's a positive."
After three days of unusually cold weather, the teams managed some effective running in warmer conditions later in the week.
McLaren took advantage of the opportunity to finally manage a serious amount of laps after reliability problems affected them on the first two days.
McLaren driver Stoffel Vandoorne, who did more than 100 laps before handing over to teammate Fernando Alonso, was second fastest, 0.521secs off Hamilton.
But the Belgian was the only driver to use the fastest hyper-soft tyre, about two seconds faster than the medium the Briton used to set his time.
Alonso was fifth fastest, 1.6secs slower than Hamilton, using the super-soft tyre.
The mix of tyres underlines the difficulty of reading true competitiveness from the list of headline times - and that's before considering that the teams will also have had varying fuel loads and engine modes, which can make significant differences to lap times.
The opening race is the Australian Grand Prix in Melbourne's Albert Park on March 25...