To investigate the relationship between tubular reabsorption of chloride and renal renin release in the isolated perfused rat kidney, perfusate renin activity was measured during substitution of either nitrate or thiocyanate for varying amounts of perfusate chloride but with maintained perfusate sodium concentration. Renin rose significantly as perfusate chloride fell; there was a sevenfold increase between perfusion with normal chloride and almost complete substitution of chloride by nitrate. With a normal perfusate chloride the addition of furosemide 10(-4) M to the perfusate also led to an increase in renin and a reduction in tubule chloride reabsorption. For all these experiments there was a significant negative correlation between renin and absolute tubular reabsorption of chloride (r = -0.68, P less than 0.001), but no such relationship with absolute sodium reabsorption. Renin release in a nonfiltering kidney, produced by elevating perfusate albumin concentration, increased approximately 40-fold. Thus increasing plasma oncotic pressure elevates renin by mechanisms additional to cessation of tubular chloride absorption. However, substitution of chloride in the perfusate by nitrate in this nonfiltering kidney did not further elevate renin release. We conclude that renin release is influenced by a signal dependent on, and inversely proportional to, chloride reabsorption in the thick ascending limb of the Loop of Henle.