James, part of the four that won Olympic gold in Beijing in 2008, trained on Thursday and attempted to allay any fears that his withdrawal from Wednesday's session was a major concern ahead of the start of the regatta at Dorney Lake on Saturday.
"We had a long trip around the athletes' village and it was quite a sunny day and I just had a bit of a raised heart rate the next morning," the 28-year-old Welshman said. "I took precautions so I just stayed in and let us the others go for a row.
"The heart rate was back down this morning. I'm feeling fine."
However, given the fact that James has a previous record of heart trouble, this latest development must be a worry to the British squad, which counts the four as its priority boat.
Also intriguing was the coaching staff's explanation of James' absence on Wednesday.
"He just wasn't feeling tops," said British Rowing performance director David Tanner, who added there were "no issues now." Men's head coach Juergen Groebler said James had been missing because of a sore throat.
Cameron Nichol temporarily took James' place in the four on Wednesday but ceded his seat 24 hours later as James, Andrew Triggs Hodge, Pete Reed and Alex Gregory took to the water in sunny conditions.
The heats of the men's four begin on Monday.
James was successfully treated for heart problems at the start of the year after he was found to be suffering from atrial fibrillation.
He was given enough time to recover to earn a place in the four for the Olympics in April ahead of Alex Partridge, who moved to the eight.
"Sometimes your training day is pretty hard and you have to take precautions," James said. "Sometimes you have to take a day off so you can recover properly, rather than get ill and take 2 or 3 days off. It's no drama."
If he remains healthy, James will be part of a crew that should be involved in one of the races of the regatta, with the Australians and a new-look American boat set to give the defending champions a real test.
Little is known of the United States — they didn't race at the three World Cup events this summer and have been kept under wraps by coach Tim McLaren — but Britain has seen at firsthand the improvements made by archrival Australia.
In the final World Cup regatta, in Munich in June, Australia led from start to finish to hand the Brits a rare loss and inflict a psychological blow just two months before the Olympics.
Triple Olympic gold medallist Drew Ginn, who is in the Australia boat, said his team's strategy of going on all-out attack from the start "scares the hell out of" Britain and believes host-nation status will weigh heavily on his rivals.
James begs to differ.
"We've made some good changes to how we row and certainly from Munich, we think we've learned from the mistakes we made there," he said.
"I think the Aussies have always wanted to beat the British, especially on our home soil. You can easily see why it's their motivation. You can't get away from it."
James believes Ginn's taunts were a sign that, in fact, Australia was the team that is worried.
"The fact that someone ... is picking us out as the opposition, that's great," he said. "I don't think it's new tactics (by Australia). He's raced like that all his career. I'm surprised he's maybe telling you this at this time. But if that's how the Aussies want to go, that's fine."