It is well established that opiate agonists alter tuberoinfundibular dopaminergic activity and consequently prolactin release. The purpose of this study was to characterize the effects of morphine on prolactin secretion and tuberoinfundibular dopaminergic neuronal activity with respect to time after administration. Additionally, the effect of an initial morphine injection on the response produced by a second injection of morphine was determined. The rate of depletion of median eminence dopamine content following synthesis inhibition by alpha-methyl-p-tyrosine was used as an index of dopaminergic neuronal activity. Male rats given a single injection of morphine sulfate (15 mg/kg, s.c.) showed a significant increase in circulating prolactin levels and had a lower rate of median eminence dopamine turnover 1 h after injection. Four hours after injection, circulating prolactin levels were similar to those in vehicle treated rats, while dopamine turnover was significantly higher than controls. When two injections of morphine sulfate (15 mg/kg, s.c.) were given 4 h apart, the stimulation of prolactin release produced by the second injection was significantly attenuated. Although this second injection caused a significant decrease in dopamine turnover, the turnover rate following this injection was significantly greater than that following the initial injection. The combination of fluoxetine and 5-hydroxytryptophan (FLX/5-HTP) caused an initial increase in prolactin secretion with plasma values returning to basal levels by 4 h. When rats were pretreated with FLX/5-HTP instead of morphine, the prolactin response to an injection of morphine 4 h later was not attenuated. Similarly a FLX/5-HTP pretreatment had no influence on a second injection of FLX/5-HTP administered 4 h later.(ABSTRACT TRUNCATED AT 250 WORDS)