Protein kinase, phosphodiesterase and adenylate cyclase of plasma membrane of adipocytes and the effect of the feedback regulator (FR) on these three enzymes was measured and compared. The basal level ratio of adenylate cyclase to phosphodiesterase to protein kinase was 1:1.9:3.0. Epinephrine and/or FR alters this ratio. FR stimulated protein kinase activity up to 3 fold in the presence of a wide range of enzyme concentrations, 5-50 mug membrane protein/tube. The concentration of FR effective for stimulation of membrane protein kinase was much greater than that needed for inhibition of adenylate cyclase and phosphodiesterases. The inhibition by FR on adenylate cyclase was the most potent effect among the 3 enzymes. 1 U (or 2 U/ml) of FR inhibited 50% of the adenylate cyclase activity in a defined system. The maximum effective concentration of FR for stimulation of membrane protein kinase was greater than 10 U/ml. Histone type 11A was the best substrate for protein phosphorylation so far observed. The FR stimulatory effect was observed at all substrate concentrations used ranging from 1-5 mg/ml. A NaF concentration curve shows that 15 mM NaF gave maximum phosphorylation. The stimulatory effect of FR was observed both in the presence and absence of NaF. Protein kinase of adipocyte plasma membrane was mainly cAMP-independent. The effect of FR (20 U/ml) in stimulation of protein phosphorylation was much greater than that of cAMP (1 X 10(-6) M). The cAMP and FR effects seemed to be additive. Preincubation of plasma membrane with FR in the absence of ATP resulted in no decrease but slight increase in protein kinase activity. A shift in protein kinase, phosphodiesterase and adenylate cyclase ratios by FR suggests the regulatory role of FR in cAMP metabolism in adipocytes.