1. The blood pressure and renal blood flow response to captopril (0·2 mg/kg, intravenously) was studied in low salt, normal, and high salt fed conscious dogs, and in a group of DOCA-salt treated dogs.
2. Mean arterial blood pressure was decreased and renal blood flow increased most in the low salt group, but significant changes were also obtained in the normal group. The high salt and DOCA-salt groups were only marginally affected by captopril.
3. When the data from all four groups of dogs were subjected to regression analysis, there was a significant relationship (r = 0·68) between the prevailing plasma renin activity and the increase in renal blood flow caused by captopril.
4. The results suggest that renal vasodilatation resulting from converting enzyme inhibition is mainly due to a decrease in the level of circulating angiotensin II, and that even in the normal conscious dog the prevailing plasma renin activity can affect blood pressure and renal blood flow.