An adaptive response has been demonstrated in certain mammalian cells wherein pre-exposure to a small radiation dose prior to a large dose ameliorates the damage induced by the second dose. We investigated whether a similar response could occur in the developing brain of the fetal rat, and if so, what the optimum interval between the two doses would be. Pregnant rats were exposed to a dose of 0.02 Gy gamma radiation at variable times (1, 3, 6, 12, or 24 h) prior to a second dose of 0.5 Gy on day 15 of gestation. Fetuses were harvested at 6 and 24 h after the second irradiation and standard cellular morphological assessments performed on the developing cerebral cortex. For number of mitotic cells, pyknotic cells, and macrophages, no significant differences were found between any of the groups that had received the priming (0.02 Gy) dose and the group that had not. Significant differences were found between fetuses harvested at 6 h and those harvested at 24 h after the final irradiation for all parameters measured. Thus, while the data were consistent with past research relating to the effects of radiation on the development of the brain of the fetal rat, no evidence for an adaptive response to radiation was found. Whether an adaptive response was indeed absent, or whether the doses and/or intervals used were simply not appropriate for demonstrating it, remains unknown.