T his study examines the effects of adrenalectomy and physiological replacement of mineralocorticoids and glucocorticoids on the cellular ultrastructure of the rat initial collecting tubule (late distal tubule). Animals were adrenalectomized (ADX) and for 10 d received by osmotic minipump either: vehicle, aldosterone (0.5 micrograms X 100 g-1 X d-1), aldosterone (2.0 micrograms X 100 g-1 X d-1), dexamethasone (1.2 micrograms X 100 g-1 X d-1), or aldosterone (0.5 micrograms X 100 g-1 X d-1) with dexamethasone (1.2 micrograms X 100 g-1 X d-1). Radioimmunoassay revealed that the low dose of aldosterone restored plasma aldosterone to control levels. The higher dose of aldosterone increased plasma levels by threefold. Morphometric techniques were used to measure membrane length of individual principal and intercalated cells in each condition. The basolateral membrane length of principal cells decreased by 35% in ADX animals. Low dose aldosterone replacement (0.5 micrograms X 100 g-1 X d-1) in ADX animals maintained membrane length at control values; at a higher level of aldosterone (2.0 micrograms X 100 g-1 X d-1) membrane length increased by 111% compared with control. Dexamethasone treatment, at a level that restored glomerular filtration rate to normal, had no effect on cellular ultrastructure. Combined aldosterone and dexamethasone replacement had no greater effect on basolateral membrane length than aldosterone alone. The length of the luminal membrane of the principal cell type was not affected by ADX or hormone treatment. Intercalated cell membrane length was not affected by ADX or hormone replacement. Thus, chronic aldosterone levels have an important, selective effect on the basolateral membrane of the principal cell. The correlation between these morphological results and the steroid hormone effects on renal electrolyte excretion, reported in the companion paper (15), suggests that basolateral membrane length is an important factor controlling the rate of sodium and potassium transport by the initial collecting tubule.