Plasma tryptophan level varied diurnally with highest concentrations at 12 noon (P less than 0.001); at that time free tryptophan level was lowest. The mean 8 h and 16 h cortisol levels were 70% higher compared to levels obtained at 24 h. Chronic administration of haloperidol and chlorpromazine led to significant increase in the plasma tryptophan (P less than 0.001 and P less than 0.1, respectively) compared to saline-treated rabbits. This change was associated with corresponding constant or reduced free tryptophan levels, and a diminution in plasma cortisol level (10 microgram/l) compared with controls (34 microgram/l). The results suggest that tryptophan in plasma is poorly transported in the brains of rabbits treated with chlorpromazine, and that treatment with haloperidol or chlorpromazine may lead to a reduced tryptophan flux into the kynurenine pathway.