Donald Trump Jr appeared to have "a very serious case of amnesia" during testimony to a congressional committee, a Democratic lawmaker has said.
The US president's eldest son testified before the US House Intelligence committee about Russia's alleged meddling in the 2016 election.
Congresswoman Jackie Speier said that Mr Trump Jr refused to answer questions about conversations with his father.
He said that those conversations were private because lawyers were present.
"My takeaway is he has a very serious case of amnesia and he was pretty non-responsive on a lot of issues that frankly, you would have a recollection of, considering it was just a year ago that many of these events took place," Mrs Speier told CNN on Wednesday night.
Mr Trump Jr told lawmakers behind closed doors that he did not tell his father about a 2016 meeting between Trump campaign officials and Russians when it happened and he declined to discuss what he did say to the president once the meeting was made public in a New York Times report, according to Democratic California Congressman Adam Schiff.
Mr Trump Jr had agreed to that meeting after being promised damaging information from the Russian government about Hillary Clinton during his father's campaign for president, his email correspondence shows.
In his first of two statements about the meeting, Mr Trump Jr made no mention of the Russian information, and instead claimed that the meeting was held to discuss the suspension of the US programme to adopt Russian children.
He later clarified to say that he held the meeting to conduct opposition research on his father's political opponent.
The Washington Post reported at the time that President Trump had dictated the first misleading statement, but Mr Trump Jr refused to tell the House committee about the phone call with his father because lawyers for both men were also on the conference call.
"He acknowledged discussing that matter with his father, but refused to answer questions about that discussion along the basis of a attorney-client privilege," Mr Schiff said.
"In my view, there is no attorney-client privilege that protects a discussion between a father and son," he added.
Republican lawmakers said they found the president's son to be forthcoming with his responses during the nearly eight-hour hearing, according to US media reports.
Mr Trump Jr also reportedly told House members about his conversations with White House Communications Director Hope Hicks regarding how he should respond to a media report about his meeting, US media reported.
He also faced questioning about his private Twitter communications with Wikileaks, the anti-secrecy website that was behind the release of a trove of hacked emails from Mrs Clinton's campaign.
Mr Trump Jr said that he had not told the president about the communications and that he views the group as an independent news organisation, US media reported.