The ductuli efferentes testis (efferent ducts) of the rat were studied to determine whether fluid and electrolyte reabsorption by the ducts is under the control of adrenal mineralocorticoids. Testicular fluid output and the rate of fluid reabsorption by the ducts were determined in sham-operated controls and in rats which had been adrenalectomized 10 days previously, adrenalectomized 10 days previously and given aldosterone therapy (75 micrograms kg-1 day-1 S.C.) from day 3 to day 10 post adrenalectomy, and in rats given injections of the aldosterone antagonist, spironolactone, for 10 days (10 mg kg-1 day-1 S.C.). The values for testicular fluid output and fluid flow from the efferent ducts (means +/- S.E.M.) in the sham-operated rats were 36.0 +/- 7.8 and 1.23 +/- 0.12 microliters h-1, respectively, resulting in an estimate of fluid reabsorption by the efferent ducts of 94.8 +/- 1.6% of the testicular fluid output. None of the treatments resulted in a significant change in testicular fluid output or in fluid reabsorption from the efferent ducts. Similarly, the treatments did not significantly alter the osmolality or electrolyte concentrations in fluid samples from the rete testis or the distal end of the efferent ducts (sham-operated values for rete testis and efferent duct fluid, respectively, were: osmolality, 311.2 +/- 1.7 and 302.7 +/- 5.7 mosmol kg-1; [Na+], 151.2 +/- 5.6 and 150.8 +/- 8.1 mmol l-1; [Cl-], 147.9 +/- 6.6 and 126.7 +/- 2.2 mmol l-1; [K+], 15.2 +/- 0.7 and 13.5 +/- 2.4 mmol l-1). It is concluded that, like the homologous proximal tubule of the metanephric kidney, fluid reabsorption from the efferent ducts is independent of adrenal mineralocorticoid control.