The effect of nicotine administered supraspinally on antinociception induced by supraspinally administered opioids was examined in ICR mice. The intracerebroventricular (i.c.v.) injection of nicotine alone at doses from 1 to 12 micrograms produced only a minimal inhibition of the tail-flick response. Morphine (0.2 micrograms), beta-endorphin (0.1 microgram), D-Pen2.5-enkephalin (DPDPE; 0.5 microgram), trans-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl) cyclohexyl] benzeocetamide (U50, 488H; 6 micrograms) caused only slight inhibition of the tail-flick response. Nicotine dose dependently enhanced inhibition of the tail-flick response induced by i.c.v. administered morphine (0.2 microgram) or beta-endorphin (0.1 microgram). The degree of enhancing effect of nicotine toward beta-endorphin-induced inhibition of the tail-flick response was greater than toward morphine-induced inhibition of the tail-flick response. However, i.c.v. administered nicotine at the same doses was not effective in enhancing the inhibition of the tail-flick response induced by DPDPE (0.5 microgram) or U50, 488H (6 micrograms) administered i.c.v. Our results suggest that stimulation of supraspinal nicotinic receptors may enhance antinociception induced by morphine (a mu-opioid receptor agonist) and beta-endorphin (an epsilon-opioid receptor agonist) administered supraspinally. However, the activation of nicotinic receptors at supraspinal sites may not be involved in enhancing the antinociception induced by DPDPE (a delta-opioid receptor agonist) or U50, 488H (a kappa-opioid receptor agonist) administered supraspinally.