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.
"However theses results proved no information as to their relevance regarding potential carcinogenic effects, or lack of such effects."
Professor Patrick Haggard, from the Institute of Cognitive Neuroscience at University College London, said: "This is a very interesting result, since it suggests a possible direct effect of mobile phone signals on brain function.
"The implications for health remain unclear. Much larger fluctuations in brain metabolic rate occur naturally, for example during thinking.
"However, if further studies confirm that mobile phone signals do have direct effects on brain metabolism, then it will be important to investigate whether such effects have implications for health."
Professor Malcolm Sperrin, director of Medical Physics and Clinical Engineering at Royal Berkshire Hospital, said: "This paper is particularly interesting in that it reports an increase in brain metabolism as a result of the use of mobile phones.
"More work is required to establish any possible link between RF energy deposition in the brain and a consequential health risk. It is reasonable to assume that the small increase in metabolism results from a deposition of energy, which may result in turn from local temperature changes or perhaps magnetic or electrical stimulation that does not involve heat at all.
"It is important to fully appreciate that no health risk is identified in this paper."