The action of adenosine on renal blood flow and kidney function in dogs was studied with bolus injections and infusion of adenosine into the renal artery. Simultaneous infusions of theophylline, 1-5 x 10(-6) mol/min into the renal artery which did not affect renal function by itself inhibited the adenosine induced vasoconstriction. From the bolus injection studies a dose response curve (DRC) was constructed. Theophylline shifted the DRC to the right in a parallel manner. pA2-pA10 was 0.98 indicating that theophylline inhibition of the adenosine effects may be interpreted as a competitive antagonism. Infusions of adenosine (0.3-1.1 mumol/min) caused a reduction of urine volume, sodium excretion and glomerular filtration rate (GFR). The decrease of GFR after adenosine infusion by 31.4% could be diminished by theophylline. It is suggested that adenosine action is based mainly on a constriction of the vasa afferentia in the outer zone of the cortex.