Joe Biden, the 77-year-old presumptive Democratic presidential nominee, pushed back strongly in an interview aired on Wednesday at a reporter who asked him if he's taken a cognitive test.
"No, I haven't taken a test," Biden told the CBS News correspondent. "Why the hell would I take a test?"
"Come on, man. That's like saying you, before you got in this programme, you take a test where you're taking cocaine or not," Biden said. "What do you think? Huh? Are you a junkie?"
President Donald Trump, 74, Biden's November opponent, has repeatedly claimed that Biden's mental abilities are slipping while touting his own capabilities.
The CBS reporter, who was interviewing the former vice president as part of a convention of Black and Hispanic journalists, noted that Trump "brags" about having "aced" a cognitive test.
"Well, if he can't figure out the difference between an elephant and a lion, I don't know what the hell he's talking about," Biden said.
"I know you're trying to goad me," he said. "But I mean I'm so forward-looking to have an opportunity to sit with the president or stand with the president and (the election) debates."
Trump recently divulged details of a cognitive test he underwent that he claimed proved he had the "mental stamina" to lead the country.
Trump called for Biden, who is currently leading in the polls, to take a similar test, because "something's going on".
"You need physical stamina," Trump said. "You need mental stamina... President Xi is sharp. President Putin is sharp. Erdogan is sharp."