A new in vitro system was developed in which renin release was studied in the absence of tubules, glomeruli, and macula densa. Rabbit afferent arterioles were isolated by microdissection and incubated in medium 199 for consecutive 15-min periods. Renin concentration of incubation media and arteriolar tissue was measured using partially purified rabbit angiotensinogen. Basal renin release rate was 0.68 +/- 0.06 ng of angiotensin I (AI) X hr-1 X arteriole-1/hr incubation of arterioles (x +/- SEM, N = 29), and remained stable for 60 min. The renin release rate was 2.76 +/- 0.22% of arteriolar renin content each hour, and there was a significant correlation between the two (r = 0.73, P less than 0.01). Renin release increased from 0.56 +/- 0.07 to 1.71 +/- 0.17 ngAI X hr-1 X arteriole-1/hr (P less than 0.01, N = 6) during exposure to isoproterenol (8.1 X 10(-5) M) and returned to basal values during the recovery period. Dietary sodium depletion resulted in a significantly greater arteriolar renin content (86.1 +/- 17.5 ngAI X hr-1/arteriole) compared with that from rabbits on a normal sodium diet (26.8 +/- 2.51 ngAI X hr-1/arteriole). However, sodium depletion did not alter the basal renin release rate suggesting that sodium depletion increased renin content in a storage pool rather than a pool contributing to basal release. It is concluded that the isolated afferent arteriole is a good model for the study of renin release in the absence of tubules, glomeruli, and macula densa.