1. Papillary osmolality and sodium and potassium concentrations were determined in rats during a control period and during escape from the sodium-retaining effect of deoxycorticosterone acetate and compared with the changes observed after acute frusemide injection.
2. During escape, papillary osmolality [554 ± 36 vs 754 ± 42 mmol/kg of papillary water (H2O), P<0.005] and papillary sodium concentration (131 ± 7 vs 182 ± 8 mmol/kg H2O, P<0.001) were significantly decreased as compared with the control values, while papillary potassium concentration remained unchanged.
3. Frusemide decreased papillary osmolality to 538 ±41 mmol/kg H2O (P<0.005), papillary sodium concentration to 125 ± 9 mmol/kg H2O (P<0.001) and papillary potassium concentration from 80 ± 2 to 69 ± 3 mmol/kg H2O (P<0.05).
4. The present results suggest that medullary portions of the distal tubule (probably the ascending loop of Henle) may represent one site of tubular sodium chloride rejection during escape from the sodium-retaining effect of deoxycorticosterone acetate.