Adrenergic regulatory mechanisms of melatonin synthesis and secretion were studied in the pigeon in vivo. Late-afternoon intraperitoneal injection of noradrenaline (NA; 1 mg/kg) resulted in a significant decrease in plasma melatonin levels in 3 h. The same effect was seen after phenylephrine treatment (1 mg/kg i.p.), indicating that an alpha 1-adrenergic mechanism may mediate the inhibition. Propranolol treatment had no effect on plasma melatonin levels, supporting this concept. Detomidine (1 mg/kg i.p.), an alpha 2-adrenergic agonist, increased melatonin levels. This stimulatory effect was blocked by yohimbine, an alpha 2-adrenergic antagonist. However, yohimbine alone had no effect on the plasma melatonin levels, suggesting that alpha 2-adrenergic transmission is not primarily responsible for the nocturnal stimulation of melatonin synthesis and secretion in the pigeon.