Oxytocin and bovine neurophysin I (bNpI) were estimated by radioimmunoassay in jugular vein plasma which was collected continuously from 18 bulls. No release of peptides was observed during successive matings with a cow in oestrus or during successive mountings on a cow with ejaculations into an artificial vagina. Stimulation with an electro-ejaculator or, to a smaller extent, massage of the seminal vesicles and ampullae per rectum caused an increase of oxytocin accompanied by a release of bNpI. It is speculated that the release of these peptides is due to stimulation of afferent pelvic nerves in the rectal wall. Basal molar ratios of bNpI/oxytocin in the plasma were highly variable, often showing a large excess of either bNpI or oxytocin. After the onset of peptide release induced by stimulation, molar ratios approached 1:1. This might indicate that hormone release is by exocytosis. Basal bNpI does not provide a good reflection of the oxytocin level.