Cold acclimation increases the thermogenic capacity of brown adipose tissue (BAT) and this is associated with both a general hypertrophy of the tissue and a selective increase in the proportion of a specific 32,000 molecular weight, GDP-binding protein in the mitochondrial membrane. This protein regulates the activity of the major thermogenic pathway of BAT. Although noradrenaline mediates the acute thermogenic response to cold, repeated injections of this hormone fail to mimic cold acclimation in that they do not increase the proportion of 32,000 molecular weight (32K) protein. However, noradrenaline is rapidly metabolised in vivo and effective levels would only be maintained over a short period of time following administration of the hormone. We have therefore investigated the effects of two long-acting sympathomimetic compounds: fenoterol, a beta-adrenoceptor agonist, and ephedrine, which causes release of noradrenaline from nerve terminals. Chronic treatment with both of these compounds increased the GDP-binding ability of BAT mitochondria and the proportion of 32K protein as measured by gel electrophoresis. We suggest that prolonged stimulation of beta-adrenoceptors, probably in BAT, but possibly also in other tissues, is sufficient to produce a selective increase in GDP-binding protein in BAT, and it is unnecessary to invoke any factor that is not a consequence of beta-adrenoceptor stimulation to explain the effect of cold.