The respiratory (thermogenic response of brown fat cells has been investigated for differentiation between alpha- and beta-adrenergic components. The relative sensitivity of the cells generally followed the pattern of the EC50 for isoprenaline less than norepinephrine = epinephrine much less than phenylephrine and the response to all these agonists was much more sensitive to propranolol than to phentolamine. Based on these criteria the response was primarily beta 1. However, the biphasic nature of the dose-response curves and the antagonist inhibition curves indicated additionally the presence of an alpha-component. Inhibition studies demonstrated the IC50 series: prazosin less than phentolamine less than yohimbine, indicating that the alpha-component is of the alpha 1-subtype. The effects of selective alpha- and beta-stimulation were additive. The maximal oxygen consumption of isolated hamster brown fat cells was composed of an 80% beta 1 adrenergic component and a 20% alpha 1 adrenergic component. Different mechanisms (beta 1 through cyclic AMP and alpha 1 possibly through Ca2+) and perhaps different purposes (e.g. short-term and long-term regulation, respectively) may explain the coexistence of two stimulatory adrenergic responses in one cell type.