1The N6-2'-O-dibutyryl derivative of adenosine 3',5'-monophosphate (db cyclic AMP) has been micro-injected into the third cerebral ventricle of the unanaesthetized, restrained mouse and the effects on body temperature and thermoregulatory activities observed. 2 Db cyclic AMP (4, 16 and 32 jg) injected intracerebroventricularly produced hypothermia when compared with temperature responses to sodium n-butyrate (6.8 jg). 3 Hypothermia induced by db cyclic AMP in mice was associated with a fall in oxygen consumption together with behavioural and autonomic heat loss activities but not cutaneous vasodilatation. The effects on rectal temperature and oxygen consumption were dose-dependent. 4 The falls in rectal temperature and oxygen consumption induced by db cyclic AMP (4 ,ug) were decreased by elevation of the environmental temperature from 22 to 32°C and abolished at 36°C. 5 It is concluded db cyclic AMP may inhibit central events mediating the rise in metabolic heat production in mice upon exposure to cold environments.