The purpose of these experiments was to investigate the relationship between renin secretion (the product of renal plasma flow and renal venous minus arterial plasma renin) and distal tubular Na* concentration and load (the product of Na* concentration and tubular fluid flow rate) in pentobarbital-anesthetized rats. Induction of saline diuresis decreased renin secretion (from 274 ± 82 to 56 ± 18 ng/hr per min per kg, n = 6 rats, P < 0.03) and increased Na* load (from 494 ± 40 to 1179 ± 181 pEq

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... in, n -13 tubules, P < 0.004). Induction of mannitol diuresis also decreased renin secretion (from 226 ± 62 to 21 ± 12 ng/hr per min per kg, n -6 rats, P < 0.03) and increased Na + load (from 479 ± 68 to 969 ± 147 pEq/min, n •» 13 tubules, P < 0.004). Na 4 concentration was increased by induction of saline diuresis (from 59 ± 2 to 79 ± 5 mEq/liter, P < 0.004) and decreased by induction of mannitol diuresis (from 66 ± 5 to 49 ± 4 mEq/liter, P < 0.004). Na* concentration, Na* load, and renin secretion rate were stable over time in a third group of rats (controls). The acute changes elicited by induction of osmotic diuresis were not consistent with a relation between Na* concentration and renin secretion, but these changes were consistent with a reciprocal relation between Na* load and renin secretion.