The effects of theophylline ethylenediamine (15 mg/kg i.v.) on plasma renin activity (PRA), diuresis, creatinine clearance and plasma and urinary adenosine 3',5'-monophosphate (cyclic AMP) concentrations were studied in urethane-anaesthetized rabbits with and without pretreatment with indomethacin (5 mg/kg i.v.). Theophylline induced a several-fold increase in urinary sodium and water excretion, raised PRA from 76 +/- 15 TO 239 +/- 83 (S.E.M) ng/ml/h (p less than 0.05) and increased urinary cyclic AMP excretion from 0.21 +/- 0.04 to 0.50 +/- 0.08 nmol/min/kg/kidney (p less than 0.05) without any change in arterial plasma cyclic AMP concentration. The ratio between the clearance of creatinine and cyclic AMP was unchanged. After indomethacin pretreatment the theophylline-induced natriuresis was significantly reduced (p less than 0.05), while the effects of theophylline on PRA and urinary cyclic AMP excretion were unchanged. Under the present experimental conditions, indomethacin reduces renal prostaglandin biosynthesis by over 95%. our results indicate that prostaglandins might be concerned with the natriuretic effects of theophylline but not with the effect on PRA.