In the present investigation, the effect of recombinant (BST) and pituitary-derived (bGH) bovine somatotrophin on progesterone and oxytocin release was examined. Individual copora lutea (CL) were obtained from cows at different stages of the oestrous cycle (days 5-7, 8-12 and 15-18) and also from early pregnancy (days 60-120) and were implanted with an in vitro microdialysis system (MDS). Perfusion with BST for 60 min (0.05, 0.5 and 5 mumol/l) induced a dose-dependent stimulation of progesterone release. Release of oxytocin from CL was significantly stimulated by BST at all dose levels. BST (0.5 mumol/l) stimulated progesterone release most during the early and mid-luteal phases and oxytocin release especially during the early luteal stage (days 5-7) of the oestrous cycle. CL from early pregnancy (days 60-120) treated with BST showed a significant response in progesterone and oxytocin release. bGH showed comparable effects. Our results suggest that somatotrophin acts directly on the secretory function of bovine CL in the MDS, specifically during the early luteal stage (days 5-7) of the oestrous cycle and early pregnancy (days 60-120). Somatotrophin may therefore have physiologically relevant effects associated with the development and maintenance of luteal function.