Oxytocin, vasopressin and neurophysin-like immunoreactivity have been identified and measured by radioimmunoassay in extracts of human and rat testis and human fetal adrenal tissue. The authenticity of these polypeptides has been confirmed by their behaviour on high performance liquid chromatography. The concentrations of the hormone were too great to be explained by known circulating levels of the polypeptides, and their presence in steroid secreting organs suggests a possible role for them in steroidogenesis. The peptides may be taken up and concentrated by the tissues but the co-localisation of neurophysins with the hormones points towards local synthesis.