Insulin inhibition of lipolysis in the presence of forskolin was reversed by a four hour exposure of adipocytes to pertussis toxin. In contrast, the antilipolytic action of insulin against lipolysis due to theophylline was unaffected by pertussis toxin as was the ability of insulin to lower cyclic AMP in the presence of either forskolin or theophylline. The stimulation of adenylate cyclase by norepinephrine in crude plasma membranes obtained from rat adipocytes was inhibited by N6-(Phenylisopropyl)adenosine (PIA) and abolished by pretreating rat adipocytes with pertussis toxin. The stimulation of glucose metabolism by insulin was not altered by pertussis toxin pretreatment of rat adipocytes. These findings suggest that pertussis toxin selectively abolishes the antilipolytic effect of insulin in the presence of forskolin through a cyclic AMP independent mechanism.