Ectopic expression of human or bovine growth hormone (GH) genes in 6 lines of transgenic mice was associated with a significant increase in plasma corticosterone levels. Elevated corticosterone levels were detected in both sexes under basal conditions and after ether stress. The adrenal activity of 3 beta-hydroxysteroid dehydrogenase was measured in two of these lines and was found to be significantly increased in transgenic animals. Plasma corticosterone levels in transgenic mice did not correlate with known differences between the various lines in male and female fertility or in the life span.