Male rats have about two times as many steroid-responsive vasopressin- immunoreactive (AVP-ir) neurons in the bed nucleus of the stria terminalis (BST) as female rats. This sex difference does not depend on differences in circulating hormone levels, since it persists in males and females that are treated with similar levels of testosterone. To analyze the cellular basis of this sex difference, we compared the effects of testosterone and its metabolites on AVP mRNA expression in the BST of males and females that were gonadectomized at 3 months of age. When rats received implants of Silastic tubing filled with testosterone, males had more cells that were labeled for AVP mRNA and more labeling per cell than females. When, in a second experiment, rats received implants of either empty tubing, or tubing with dihydrotestosterone (DHT), estradiol (E), or E plus DHT, hardly any labeled cells were found in rats with empty implants. E treatment significantly stimulated AVP mRNA expression in both sexes, but significantly more so in males, which had more cells that were labeled for AVP mRNA and more labeling per cell than females. DHT treatment by itself did not stimulate AVP mRNA expression, but when given in combination with E, it significantly increased the number of cells over that of animals treated with E alone. This increase was seen in males only. However, in both sexes, it increased the labeling per cell over that of animals treated with E only, but more so in males than in females