Lando Norris set the quickest time of the day late in proceedings, though logged more than 100 laps across the eight hours of running.
Though the Brit had sampled the MCL36 during a promotional outing earlier in the week, Wednesday was the first opportunity to get any prolonged seat time.
“I'm really happy to be back behind the wheel,” he said followed the day's running.
“It's nice to be in the car again and get to push the limit. It's been good for me to get used to this new car, because I've driven the same regulations for my whole F1 career.
“It's nice to have a change and it was a productive day,” he added.
“We learned a lot of things for the team, and we got all the information we wanted to.
“We can now compare it back to the simulator and the wind tunnel. A good day, and a good start to this test.”
Norris offered the day's only minor interruption, when his McLaren stopped just across the white line which denotes pit exit.
While the track remained green, pit exit was closed temporarily as his car was pulled back and reset in the garage before resuming the session.
That saw him chalk up more than a grand prix worth of distance, affording the team important data early on as it looks to confirm its design choices.
“We're not taking any notice of lap times at the moment, it's all about learning and following our own directions and priorities,” explained James Key, McLaren's executive director, technical.
“We gathered some important data points during this first day of testing and learned a great deal, both to carry forward for the rest of this test and also to use as we develop the car further.
“Like many teams, we undertook initial runs with aero rakes on the car this morning which provided some useful initial correlation data,” he continued.
“Aerodynamics is the biggest change with the '22 cars so it is important to take some basic initial measurements at an early stage.
“Through the rest of the day, we did a fair amount of set-up and exploratory work, beginning to understand the car and the tyres a step at a time.
“I think we can be pleased with the number of laps we've covered with Lando today and look forward to going again with Daniel tomorrow.”
Daniel Ricciardo steps into the car on Thursday before he and Norris share duties on the final day of running.
Testing in Barcelona resumes at 09:00 local time (19:00 AEDT).