The authors investigated the effects of spinal and supraspinal administration of the benzodiazepine receptor agonist midazolam alone and with opioids on tests of nociception (tail-flick and hot-plate tests) and motor function (catalepsy) in rats. At the spinal level, the dose-response curves for peak effect and area under the curve for morphine were shifted to the left (indicating potentiation) by a submaximal dose of intrathecal (i.t.) midazolam (20 micrograms) in both nociceptive tests. Additionally, 2.5 micrograms of i.t. midazolam, a dose having no effect when given alone, increased antinociception in both tests when given with i.t. morphine. Isobolographic analysis confirmed that i.t. injection of midazolam potentiated antinociception induced by i.t. morphine. At the supraspinal level, intracerebroventricular (i.c.v.) injection of 4 micrograms of midazolam inhibited morphine antinociception, i.e., the dose-response curve for morphine in the hot-plate test shifted to the right. Midazolam did not affect morphine antinociception in the tail-flick test. Catalepsy occurred only when the highest doses of i.t. or i.c.v. morphine or midazolam were injected alone. The differing effect of midazolam on morphine-induced antinociception suggests that different mechanisms are involved in the spinal cord and brain.