The ability of dynorphin and beta h-endorphin (beta h-EP) to stimulate prolactin (PRL) release in male rats was examined. Rats were injected intraventricularly with either 1 or 10 microgram dynorphin, 1 or 10 microgram beta h-EP, 10 microgram dynorphin together with 20 microgram naloxone (NAL), or an equivalent volume (13 microliter) of saline. Dynorphin caused a dose-dependent release of PRL which was significant 10 min after injection but not at later sampling times. beta h-EP also increased serum PRL levels and maintained elevated PRL levels for at least 60 min. NAL, a specific opiate antagonist, completely blocked dynorphin-induced PRL release. These results indicate that dynorphin is a potent stimulator of PRL release, but its stimulatory activity is transient relative to the activity exhibited by beta h-EP. Furthermore, dynorphin's action is specific since NAL completely blocked dynorphin induced PRL release.