The rodents in the studies were exposed to radiation nine hours a day for two years — far longer even than heavy users of cellphones. For the rats, the exposures started before birth and continued until they were about 2 years old.

Some 2 to 3 percent of the male rats exposed to the radiation developed malignant gliomas, a deadly brain cancer, compared to none in a control group that received no radiation. Many epidemiologists see no overall rise in the incidence of gliomas in the human population.

The study also found that about 5 to 7 percent of the male rats exposed to the highest level of radiation developed certain heart tumors, called malignant schwannomas, compared to none in the control group. Malignant schwannomas are similar to acoustic neuromas, benign tumors that can develop in people, in the nerve that connects the ear to the brain.

The rats were exposed to radiation at a frequency of 900 megahertz — typical of the second generation of cellphones that prevailed in the 1990s, when the study was first conceived.

Current cellphones represent a fourth generation, known as 4G, and 5G phones are expected to debut around 2020. They employ much higher frequencies, and these radio waves are far less successful at penetrating the bodies of humans and rats, scientists say.

In June, at a meeting of scientific counselors to the toxicology agency, Donald Stump, one of the members, worried that the study “will be vulnerable to criticism that it was conducted using outdated technology.” The challenge, he added, is how to move forward with experiments that are large enough to be significant yet nimble enough t o keep pace with the rapidly evolving devices.

The toxicology agency is building smaller exposure chambers that will let it evaluate newer technologies in weeks or months, instead of years. These future studies are to focus on measurable physical signs, or biomarkers, of potential effects from radio-frequency radiation, including DNA damage, which can be detected much sooner than cancer.