Periaqueductal gray matter (PAG) has been shown to be one of the sites in the central nervous system where microinjections of morphine strongly inhibit intestinal transit. To investigate the nature of opioid receptor populations involved in this central effect, selective opioid agonists, FK 33824 for mu, DALA for delta, dynorphin for kappa and tentatively beta-endorphin for epsilon, were microinjected in all PAG areas previously identified as morphine-sensitive for intestinal inhibition. The PAG-induced inhibition of intestinal transit appears to be mediated mainly by mu receptors and possibly by epsilon receptors. kappa and delta receptors seem not to be involved.