Serotonin and dopamine neurons have been shown to exert a stimulatory and inhibitory control, respectively, over pituitary release of β-endorphin-like immunoreactivity (β-END-LI). In the present study we sought to determine whether an interaction exists between these two reciprocal mechanisms regulating β-END-LI in the rat. The intraperitoneal (i.p.) administration of 5 mg/kg quipazine, a serotonin receptor agonist, or 2.5 mg/kg haloperidol, a dopamine receptor antagonist, each elevated circulating levels of β-END-LI 5-fold over control levels by 30 min post-injection. Pretreatment (1 h) with 5 mg/kg, i.p., cinanserin, a serotonin receptor antagonist, completely blocked the quipazine-induced rise in β-END-LI without affecting the elevated levels of β-END-LI in haloperidol-treated animals. Conversely, pretreatment (2 h) with 1 mg/kg, i.p., bromocriptine, a dopamine receptor agonist, had no effect on quipazine-induced release of β-END-LI but did completely prevent the rise in plasma β-END-LI due to haloperidol treatment. Gel filtration chromatography revealed that quipazine and haloperidol treatments elevated plasma levels of both β-END-size immunoreactivity and β-lipotropin (β-LPH)-sized immunoreactivity though to different relative degrees. However, since circulating levels of β-LPH serve as a marker for anterior lobe (AL) β-END-LI secretion, serotonin and dopamine appear to exert stimulatory and inhibitory control, respectively, over AL β-END-LI release. Further, the quipazine-induced rise in total plasma β-END-LI primarily resembled β-LPH in size and was blocked by cinanserin but not bromocriptine pretreatment. And conversely, bromocriptine but not cinanserin prevented the haloperidol-induced rise in circulating β-END-LI. In comparison to quipazine, haloperidol had a considerably more marked effect on plasma levels of β-END-sized immunoreactivity. Together, these findings support the conclusion that dopamine and serotonin neurons exert reciprocal and independent control over pituitary β-END-LI secretion in vivo.