Abstract 1. 1. Central serotoninergic participation in prolactin (PRL) secretion regulation is a well-established event Furthermore, S-HT may participate in the mediation of stress-induced PRL release. 2. 2. The authors investigated the effect of the blockade of 5-HT 3 receptors on the PRL release induced by immobilization stress in male rats. 3. 3. Pretreatment with the 5-HT 3 receptor antagonist, ondansetron (0.10 – 1.0 mg/kg i.p.), inhibited about 50% of the PRL response to immobilization stress. A lower dose of ondansetron (0.05 mg/kg i.p.) had no effect on the PRL response. No dose-response inhibition was observed. 4. 4. It is concluded that serotoninergic activation related to the mediation of the stress-induced PRL response involves the 5-HT 3 receptors.