Fernando Alonso has completed his test of an Andretti-run 2018 Indycar at Barber Motorsports Park.
Alonso conducted the running in the machine last used by Carlos Munoz at the 2018 Indianapolis 500, converted to road course spec.
McLaren has been keen to keep the test as private as possible, with no media or fans allowed to attend.
Autosport understands that series chassis builder Dallara had no presence at the test and IndyCar's support was only minor and said to be nothing more than "procedural".
Despite threats of poor weather caused by Hurricane Gordon, Alonso's running was believed to be unhindered.
"It was a good day, a fun day," said Alonso. "I love to test new cars and to test the Indycar on a road course is something special.
"I've been lucky enough to test it in wet conditions, in intermediate conditions and in dry conditions, so overall I had a good feeling on every type of track.
"The weather was good for us today - a little bit of wind in the afternoon but overall a positive day."
The test was Alonso's first experience of a road course in an Indycar, with his only previous running coming at the Indianapolis Motor Speedway's oval.
He did, however, attend the Barber weekend last year as he became more familiar with the IndyCar paddock ahead of his Month of May.
"Probably my instinct will tell me [I prefer the] road course, because it's what I've been used to doing all my life, but the Indy 500 was an amazing experience, so 50/50," Alonso said.
"I think I love the way the car feels on a road course, but I love the way you compete on ovals, timing the tows, traffic and all the overtaking manoeuvres I think are a little bit easy on the ovals, so in terms of track action I loved the Indy 500."
Alonso added that the test was on his "bucket list", with Autosport understanding that Andretti had booked several test dates in 2018, just in case the Spaniard became free to try an Indycar.
"It was something [the test] that I was looking for last year already," he said. "I had some options to test the car on a road course after the Indy 500.
"We didn't find the time but this year it's definitely happened now and I'm happy for this.
"I love being behind a steering wheel, and definitely a new car, a new experience, learning a lot of things from the team, the engineers, everyone, so a happy day."
Alonso has previously stated that the test will have no influence on his racing future, with the McLaren driver set to exit F1 at the end of 2018.
Autosport has been told that Dallara has had no direct approach from McLaren over a supply of chassis, with multiple sources suggesting that McLaren's "fluid" IndyCar future is more likely to be an affiliation with an existing team.
Andretti is the favoured outfit, having worked with McLaren at the 2017 Indy 500 as part of Alonso's triple crown bid.
McLaren had previously approached Scott Dixon and Will Power over a drive for its team, and McLaren CEO Zak Brown has stated that the team's potential IndyCar future would be decided this month.
McLaren had prioritised sorting its 2019 F1 line-up and it completed the decision this week by promoting Lando Norris to partner new signing Carlos Sainz Jr.