The prolactin release caused by salsolinol (a derivative of dopamine, DA) in rats could be prevented by its structural analogue 1-methyl-3,4-dihydroisoqinoline (1-MeDIQ). To study the participation of salsolinol in the neural stimulatory mechanism of prolactin release in lactating sheep, we tested whether 1-MeDIQ, acting at the central nervous system (CNS) level, would diminish basal prolactin release and reduce prolactin surge induced by suckling. A series of intracerebroventricular (icv) infusions of 1-MeDIQ (5 x 60 microg/60 microl/30 min, at 30-min intervals) were performed in nursing ewes (n=8) during the fifth week of lactation. Additionally, by combining these infusions with push-pull perfusion, we studied the concentration of dopaminergic components, i.e., salsolinol, DA and 3,4-dihydroxyphenylacetic acid (DOPAC) within the infundibular nucleus/median eminence (IN/ME) in four of the ewes. Treatment with 1-MeDIQ significantly (P<0.001) reduced either the basal prolactin release during the non-suckling period or the suckling-induced prolactin surge. Specifically, the suppressive effect occurred gradually, affecting both the duration and amplitude of the prolactin surge. In the control ewes, the perfusate salsolinol concentration increased significantly (P<0.001) during suckling, while in the ewes treated with 1-MeDIQ only vestigial amounts of this compound were found during the non-suckling period. No DA was detected in the perfusates collected from the IN/ME of control and 1-MeDIQ-treated groups and no significant differences were found in the DOPAC concentrations between these groups. In conclusion, 1-MeDIQ is able to inhibit prolactin secretion in lactating sheep, acting at the CNS level. In addition, one of the way of 1-MeDIQ action may be directed to the local salsolinol release within the mediobasal hypothalamus.