The effects of hyperthyroidism and hypothyroidism on the metabolism of adipocytes isolated from rat brown adipose tissue were as follows: The yield of brown adipocytes was 65% less in the hypothyroid as compared to the control rats. No change in cell recovery was observed in the hyperthyroid group as compared to controls but there was a 65% increase in cell volume. The stimulation of respiration by isoproterenol and forskolin was markedly greater in cells from hyperthyroid as compared to euthyroid rats. In the adipocytes from hypothyroid rats, respiration and lipolysis were reduced but there was no defect in stimulation of adenylate cyclase by forskolin or isoproterenol as compared to euthyroid controls. The effects of different thyroid states on respiration did not correlate with changes in adenosine 3':5'-cyclic phosphate (cyclic AMP) or lipolysis. Phenylephrine in the presence of alprenolol or octanoate were as potent stimulators of respiration as 10 mumol/L isoproterenol in adipocytes from hypothyroid rats. In cells from hyperthyroid rats phenylephrine was twice, octanoate three times, and isoproterenol 16 times more effective in stimulating respiration than in cells from hypothyroid rats. These data indicate that thyroid status regulates beta-catecholamine and forskolin stimulation of respiration in brown adipocytes.