Calcitonin (CT) secretion is not exclusively under the control of calcium levels in the plasma, but also depends on the sympathetic-adrenergic tone. In previous experiments we stressed out the possible role played by 5-leu enkephalin (5-LE) in the nervous regulation of the CT secretion. Intracerebroventricular (i.c.v.) 5-LE in doses of 100 micrograms could act at a central level through a mechanism independent from the noradrenergic pathways, since i.c.v. 6-OHDA and propranolol could not interfere with its stimulatory effect on the CT content of the thyroid. In the present experiments, performed in anaesthetized Wistar-Bratislava rats, we studied the involvement of mu and delta receptors in the central effect of 5-LE on the CT content of the thyroids and the CT levels in the plasma. These parameters were measured in parallel, by means of a competitive radioimmunoassay with double antibodies (Peninsula Lab.). 5-LE probably bound to both mu and delta receptors, since its effect on the CT secretion was reversed by 13 micrograms of naloxone (i.c.v.). A partial blockade, comparable to naloxone, was noticed after 382 micrograms of CTOP (i.c.v.), a specific antagonist of the mu receptors--stressing out their involvement in the stimulatory effect of 5-LE on the CT secretion. 250 Ug of ICI 174864 (i.c.v.), a selective antagonist the the delta receptors completely blocked the stimulation induced by 5-LE on the CT secretion to values significantly lower as compared to the controls and even to the sympathectomized group. This suggests the tonic role played by enkephalins in the CT secretion, through the central activation of delta receptors.(ABSTRACT TRUNCATED AT 250 WORDS)