A study by the National Institutes of Health in the US suggests that mobile phones could have an effect on the brain.
They reported higher sugar use in the brain, a sign of increased activity, after 50 minutes on the phone.
The research published in the Journal of the American Medical Association said the clinical significance was unknown.
Experts in the UK said there was no suggestion of a health risk.
Since the boom in mobile phone use, there has been considerable interest in the effect on the body.
The largest study on 420,000 mobile phone users in Denmark, has not shown a link between phone use and cancer.
This small study on 47 people investigated the effect of magnetic fields (RF-EMFs) coming from a phone's antenna.
It suggests that brain activity is affected, but cannot draw any conclusions about possible health implications.
Mobile phones were attached to both ears of each participant. One phone was off, the another was on but muted so the person could not tell the difference.
Their brains were then scanned to detect changes in glucose use, which increased by 7% in parts of the brain close to the antenna.
The researchers conclude that "the human brain is sensitive to the effects of RF-EMFs from acute cell phone exposures.