Performance in a vigilance task and the associated neuroendocrine changes during the performance in the task were examined in healthy subjects and in three groups of male heroin-addicts both before undergoing rehabilitation programmes and at various intervals from withdrawal (5 days, 1 to 2 mon, and 3 to 48 mon, respectively). Plasma levels of ACTH, beta-endorphin (EP) and prolactin (PRL) were measured every 10 min before and during performance. In drug addicts, simple reaction times never showed any significant difference as compared to control values. Despite similar baseline levels, ACTH exhibited a markedly depressed response to psychological testing in drug-addicts as compared to controls. Whereas a three-fold increase in ACTH was observed in 'normal' subjects during the performance (from 17 to 54 ng/l), mean values from drug-addicts remained unchanged. EP levels showed a wide scatter of individual values and inconsistent time courses over performance testing: after short-term abstinence, EP showed a three-fold increase over baseline control values but, contrary to what seen in 'normal' subjects, no changes over time were recorded. After long-term abstinence, basal EP was close to control values, but its increase during the testing period was still blunted. PRL levels decreased over the testing period both in controls and in heroin addicts. Thus, despite the lack of obvious signs of neurotoxicity, drug abusers show neuroendocrine changes consistent with a long-lasting selective impairment of the hypothalamic modulation of pituitary secretion.