The effect of i.p. injection of sodium barbitone and chlorpromazine (25 mg/100 g body wt) on the levels of NE, 5-HT and body temperature was investigated in the brain regions of the field rat and the guinea-pig. Injection of the field rat with barbitone sodium or chlorpromazine provoked a general increase in the NE and 5-HT concentrations of the various brain parts. In guinea-pig variable changes were observed. Following injection with either of the two drugs, hypothermia was induced in the two animals at all of the time intervals examined.