Cold stress led to elevated plasma glycerol and free fatty acid (FFA) levels in 12 airventilated puppies. In contrast to persisting high plasma glycerol and FFA levels in the control group, marked falls of these plasma constituents occurred in six animals made hypoxic. Returning the hypoxic puppies to air breathing resulted in partial recovery of prehypoxic elevations of plasma glycerol and FFA. These observations in puppies indicate that lipolysis stimulated by cold stress was inhibited by hypoxia. Since FFA is a major fuel for heat production, reduced lipolysis at low oxygen tensions may contribute to impaired thermogenesis in young hypoxic mammals.