The study was aimed at elucidating the possible participation of L-type Ca2+ channel in the acute analgesic effect of an opiate and the development of tolerance to this action. Sufentanil, a selective mu agonist, and two dihydropyridines, the Ca2+ antagonist nimodipine and the Ca2+ agonist Bay K 8644, were selected. The tailflick test was used to assess the nociceptive threshold. In naive rats, nimodipine (200 micrograms/kg) potentiated the analgesic effect of sufentanil reducing the ED50 from 0.26 to 0.08 microgram/kg. Similar results were observed with its (-)-enantiomer Bay N 5248, while the (+) enantiomer Bay N 5247 was ineffective. Tolerance to the opiate was induced by chronic subcutaneous administration of sufentanil with minipumps (2 micrograms/h, 7 days). In these conditions the dose-response curve to sufentanil was displaced to the right and the ED50 was increased to 1.49 micrograms/kg. In tolerant rats, nimodipine preserved its potentiating ability and prevented the displacement to the right of the sufentanil dose response-curve (ED50 = 0.48 microgram/kg). When nimodipine was pumped (1 microgram/h, 7 days) concurrently with sufentanil, the development of tolerance to the opioid was not disturbed. However, the expression of tolerance was abolished and even the effect of acutely administered sufentanil was markedly potentiated (ED50 = 0.03 microgram/kg). Similar experiments were performed with Bay K 8644. In naive rats, Bay K 8644 at a low dose (20 micrograms/kg) that behaves as a calcium agonist, antagonized the analgesic effect of sufentanil (ED50 = 0.58 microgram/kg), whereas at a high dose (200 micrograms/kg) it potentiated this action (ED50 = 0.15 microgram/kg).(ABSTRACT TRUNCATED AT 250 WORDS)