Foetal rat pancreas (20-day postcoitum) was grown in organ culture using a natural media (serum and chick embryo extract). The media was supplemented with several adrenal and gonadal steroids; precursors, glucocorticoids, mineralocorticoids, estrogens, androgens and progesterone. The effects of the steroids on the pancreatic explants were quantitated biochemically by analysis of amylase and insulin in the incubated culture media and in the explanted pancreatic tissue, and morphologically by quantitative morphometric analysis. Corticosterone, hydrocortisone, aldosterone and dexamethasone fully preserved the acinar cell component. The effect was concentration-dependent. Deoxycorticoste-rone and cortexolone had intermediate effects even at the highest concentration. Gonadal steroids had no effect on the acinar or islet component in this culture system. Some of the steroids inhibited the selective islet growth seen in the control explants as well as inhibiting insulin secretion. The relationship between these data and other work in this area is summarised. In addition, the possible implications of these data relating to normal in vivo pancreatic development are discussed.