Exposure of a rat to cold (4 °C) is known to induce a biphasic change in brown adipose tissue mitochondria, believed to reflect alterations in the thermogenic, purine nucleotide sensitive proton conductance pathway; an initial rapid and large increase in purine nucleotide binding, unaccompanied by any marked change in the 32 000 polypeptide which is the binding site for these nucleotides, is followed by a slower increase in concentration of the 32 000 polypeptide accompanied by a further increase in purine nucleotide binding. The initial rapid effect of cold stress was mimicked by intravenous infusion of noradrenaline; neither the effect of cold exposure for 24 h nor the effect of intravenous infusion of noradrenaline was prevented by cycloheximide. In contrast, the slow adaptive changes in the mitochondria did not occur in response to prolonged (2 weeks) treatment with noradrenaline, although such treatment did induce the expected tissue hypertrophy accompanied by mitochondrial proliferation. Cold-induced (1 week) increases in purine nucleotide binding and 32 000 polypeptide were not prevented by oxytetracycline. The increase in purine nucleotide binding during the 2nd day of cold exposure was prevented by cycloheximide. The effect of cycloheximide on the increase in the 32 000 polypeptide could not be assessed because sufficiently long-term experiments could not be done with this compound. Thus, the initial response to cold stress appears to involve unmasking of mitochondrial proton conductance pathway sites, most probably mediated by noradrenaline. The slower adaptive response occurs in parallel with tissue hypertrophy, which itself may be mediated by noradrenaline, and appears to require cytosolic but not mitochondrial protein synthesis. However, the changes in mitochondrial composition which result in an increased concentration of proton conductance pathway sites are not mediated by noradrenaline.