PRL secretion was evaluated in 11 patients with Cushing's disease (7 women, 4 men). Basal morning levels were elevated when compared to normal subjects. PRL reactivity to TRH and hypoglycemia was normal in most patients. The 24-h mean PRL levels were elevated, with partial or total alteration of the nyctohemeral rhythm. Cure of Cushing's disease by selective pituitary adenomectomy restored a normal PRL secretion.