The importance of sex differences in major affective diseases such as depression is providing a new focus for investigating the interactions between sex, sex steroids and antidepressants. In this study, we examined the acute effects of sertraline, a selective serotonin reuptake inhibitor (SSRI) and imipramine, a tricyclic antidepressant (TCA) on the endocrine endpoints, adrenocorticotropin (ACTH) and cortisol secretion in gonadectomised male and female sheep. Each sheep was treated with an acute subcutaneous (s.c.) injection containing vehicle, sertraline (5 and 10 mg/kg), or imipramine (10 mg/kg) in the presence and absence of sex steroid replacement. In males, SSRI treatment consisted of testosterone (2 x 200 mg s.c. pellets), and in females, estradiol (1 cm s.c. implant) plus an intravaginal controlled internal drug release device containing 0.3 g progesterone. ACTH and cortisol were measured in jugular blood. Female sheep responded to sertraline treatment with dose-dependent ACTH and cortisol increases that were unchanged by sex steroid replacement. In castrated males, however, only the highest dose of sertraline increased ACTH and cortisol, and this increase was abolished in the presence of testosterone replacement. Imipramine affected neither ACTH nor cortisol secretion in either the sex or sex steroid condition. We conclude that the sex and sex steroid-related differences in the male and female responses to sertraline treatment may reflect sex and sex steroid dependent differences in serotonergic activation of the HPA axis. This highlights the potential significance of sex and circulating sex steroids in modulating neuroendocrine responses to antidepressants, and may have an impact on our understanding of the actions of these drugs in men and women.