Compensatory renal hypertrophy leads to an increase in the size and metabolic capacity of renal tubular cells. Increased transport and metabolic activities must be sustained by an augmented rate of energy production, which is largely dependent on mitochondrial processes. Although previous studies have suggested that mitochondria proliferate in the hypertrophying cell, the data to support this have not been convincing. This study was designed to determine whether the mitochondria of the hypertrophied renal proximal tubular cell undergo hypertrophy or proliferation. Flow cytometric analysis of proximal tubular cells obtained from the kidneys of uninephrectomized rabbits revealed an increase in cell size and RNA content compared with control cells but showed no change in DNA content and nuclear size and no evidence of entry into the S/G2/M phases of the cell cycle. Histomorphometric analysis of cortical proximal tubules revealed that although cytoplasmic volume increased, mitochondrial density remained constant, indicating that mitochondrial volume increases in proportion to the increase in cell volume. By day 14, mitochondrial volume had increased 66% above control values. Electron microscopic examination of isolated S2 proximal tubules from 5/6 nephrectomized rabbits with maximal hypertrophy revealed mitochondrial cristae which appeared to be more densely packed than that in normal cells. The size of the functional mitochondrial pool per cell was determined by rhodamine-123 fluorescence. This increased within 24 h of uninephrectomy, peaked at approximately 80% above control levels at 5 days, and remained elevated throughout the 16 days of observation. The initial increase (days 1 and 2) occurred before a measurable increase in mitochondrial volume occurred and presumably reflects an increase in mitochondrial membrane potential.(ABSTRACT TRUNCATED AT 250 WORDS)