In isolated bovine adrenal zona fasciculata cells, the use of adenosine deaminase to remove endogenous adenosine had no effect on basal or angiotensin II-stimulated steroidogenesis but enhanced ACTH 1–24-stimulated steroidogenesis over the entire dose response range without appreciable change in potency of ACTH 1–24. 8-Phenyl-theophylline, an adenosine antagonist, mimicked all of the actions of adenosine deaminase. High concentrations (> 1 μM) of N 6-phenylisopropyl-adenosine (PIA) increased basal, angiotensin II and cyclic AMP-stimulated steroidogenesis, whilst inhibiting the ACTH 1–24-stimulated condition. PIA also increased the potency of angiotensin II approx 20-fold. These observations are consistent with the possibility that adenosine exerts effects on two different signalling systems within zona fasciculata cells.