Steroid aromatase activity was measured in homogenates of combined hypothalamic and amygdaloid specimens obtained from 17- to 21-day-old male and female rat fetuses. The fetuses were obtained both from normal mothers and mothers exposed to a regimen of stress that results in a failure of behavioral masculinization and defeminization of male offspring. Tissue samples from stressed mothers and controls were obtained during the dark phase of the day (Villanova group, days 17, 18, 19, 20 and 21 postconception). Additional samples were obtained during the light phase (Hershey group, days 17.5, 18.5, 19.5 and 20.5 postconception). The aromatase assay used, based on the measurement of tritiated water formed during an incubation with [lβ-<sup>3</sup>H] androstenedione, had a sensitivity of 10–15 fmol per tube. There was no sex difference in aromatase activity in fetuses of either control or stressed mothers. When data from the two sexes were combined, the following sfatistically significant effects were identified: (1) lower aromatase activity in stressed compared with control fetuses on days 18, 19 and 20 postconception, (2) a progressive decline in enzyme activity between days 18.5 and 20.5 postconception in the Hershey group (controls) and between days 19 and 20 in both the control and stressed fetuses in the Villanova group, and (3) an increase in enzyme activity in the stressed fetuses between days 20 and 21. No relationship was evident between steroid aromatase activity in brain and circulating testosterone levels in male fetuses as determined in a previous study. Failure to detect sex differences or an effect of testosterone on aromatase activity could be due to tissue dilution since enzyme activity may be concentrated in a few discrete nuclear regions. Measurement of enzyme activity in these regions is needed before arriving at any definitive conclusions.