Endomorphin-1 and endomorphin-2 were recently postulated to be endogenous mu-opioid receptor agonists. We have investigated the antinociceptive and antihyperalgesic effects of intrathecally administered endomorphins in cumulative doses (0.1-100 microg) on acute and inflammatory pain sensations in awake rats. In the tail-flick test, both peptides caused a dose-dependent short-lasting antinociception, except at the highest dose, which caused motor impairment also. The dose-response curves revealed the development of acute tolerance (tachyphylaxis) to endomorphin. Similarly in the carrageenan-injected paw, the endomorphins (10 microg) exerted transient antinociceptive effects. These are the first data to demonstrate decreased responsivity in models of both acute and inflammatory pain after intrathecal administration of endomorphin-1 and -2 in awake rats.