The role of adrenergic receptors in the central thermoregulatory pathways controlling the shivering activity has been studied in groups of cold-adapted (CA), warm-adapted (WA) and newborn (NB) guinea pigs, which show quantitative differences in shivering threshold. In the CA and NB animals, which normally start to shiver at lower mean body temperature levels than the WA controls, microinjection of noradrenaline (1 g in 1 l) into the noradrenaline-sensitive area of the anterior hypothalamus elicited shivering at higher body temperatures at which normally only WA animals start to shiver. Similar injections into the hypothalamus of WA animals did not induce any further shift of the shivering threshold. Microinjections of the alpha-receptor blocking agent phentolamine into the same brain area shifted the shivering threshold in all groups of animals to lower body temperatures, the shift being proportional to the injected dose of phentolamine. The CA and NB animals required higher doses of phentolamine to produce a change in shivering threshold. It is concluded that adrenergic alpha receptors are involved in the central thermoregulatory mechanisms which adjust the thresholds for the thermoregulatory reactions.