The man England hope will captain them in their Ashes defence later this year in Australia, spent an hour in the nets at the Nursery End while his team-mates piled on the runs against Pakistan during the first Test.
Ashes-winning all-rounder Flintoff, whose recovery from an ankle problem has so far seen him play two Twenty20 matches for Lancashire, was monitored by county physio Dave Roberts and Dr Peter Gregory, the England and Wales Cricket Board's chief medical officer.
After a brief massage of his troublesome left ankle, which has sidelined him since the final Test against Sri Lanka six weeks ago, after bowling at a lively pace, he then batted against members of the Lord's groundstaff for 30 minutes.
"I bowled six overs so it was my best workout yet and I'm not creaking or anything," said Flintoff, who is now due to feature for Lancashire in their County Championship match against Kent at Canterbury starting on Tuesday.
"I've played two Twenty20 matches with Lancashire, bowled three overs and two overs so far, but I wasn't bowling at full pelt today - I've still got something else remaining to give.
"I've had two tough days last week running up and down hills and it's all been building up towards bowling and geared towards playing at Canterbury on Tuesday."
But Flintoff, who England hope will be their captain in the second Test at his Old Trafford home ground on July 27, warned: "I'm not going to get ahead of myself about my fitness for the second Test because I can't be sure until I've bowled in a four-day match.
"This is all part of the recovery and the fitness work. I'm going to have another net and practice with the England lads again tomorrow (Saturday)."