Following a multistep extraction and fractionation of sheep and human milk, immunoreactive and biologically active somatostatin (SS) was demonstrated as a constituent of milk. Milk, in contrast to serum, contains only SS-14-like material and not SS-28. HPLC-purified sheep milk somatostatin was found to inhibit both basal and prostaglandin-induced release of growth hormone from cultured rat pituitary cells. The effect was dose-dependent and similar to that of synthetic SS. In order to determine whether SS or vasoactive intestinal peptide (VIP) can be synthesized by the mammary gland we have looked for SS and VIP messenger RNA in rat glands extracted on different days of lactation. No messages for SS or VIP were detected in the mammary gland, whereas, as expected, hybridization signals for alpha-casein were evident. These results suggest that somatostatin and VIP are not synthesized by the mammary gland and are probably concentrated from blood. In other experiments we have demonstrated that one hour after oral administration of labelled somatostatin to rat pups, about 95% of the neuropeptide, extracted from the stomach, is eluted exactly at the same position as the marker peptide. These results suggest that, in contrast to the adult, somatostatin remains intact in the stomach of the neonate and therefore may be absorbed in a biologically active form.