Circardian rhythms occur in hypothalamic serotonin content in the turkey. Inhibition of serotonin (5-HT) synthesis by parachlorophenylalanine (PCPA) blocks circadian variation in hypothalamic serotonin levels suggesting that the major cause of diurnal fluctuations of differing rates of synthesis of the amine. PCPA treatment also abolishes the diurnal rhythm in plasma corticosterone and elevates the basal levels of this steroid suggesting that 5-HT terminals in the hypothalamus may inhibit the release of ACTH at certain times of the day and in this fashion generate diurnal fluctuations in plasma corticosterone. Hypothalamic levels of dopamine (DA) and norepinephrine (NE) also vary diurnally, and synthesis inhibition with alpha-methyl-para-tyrosine abolishes the diurnal rhythm in NE but not DA or in plasma corticosterone. Temporal changes in release and/or degradation of DA are significant factors contributing to the diurnal rhythms of this amine in the hypothalamus. Diurnal fluctuations in plasma corticosterone do not depend on recent synthesis of catecholamines.