OBJECTIVE: To determine whether the renal growth associated with obesity is due to hypertrophy or hyperplasia. DESIGN: New Zealand white female rabbits were fed either standard rabbit chow (n 17) or chow foriti®ed with 10% corn oil plus 5% lard (n 18) for 12 ± 16 weeks. MEASUREMENTS: All rabbits were weighed, and intra-arterial blood pressures were successfully measured at the end of the study in 16 lean and 18 obese rabbits; percent water of entire kidneys (8 lean, 8 obese rabbits) and of de®ned regions of kidneys (8 lean, 10 obese rabbits) were obtained gravimetrically. Renal hemoglobin, protein and DNA was measured chemically (8 lean, 8 obese rabbits). RESULTS: Kidneys grew in size as the rabbits gained fat. In a series of 8 lean and 8 age-matched obese rabbits, weighing 3.7 AE 0.1 kg and 5.4 AE 0.4 kg (P`0.05), the kidneys were 20% larger in the obese rabbits: 15.0 AE 0.9 g vs 18.0 AE 2.5 g (P`0.05). Kidney protein was also 20% greater in the obese rabbit: 1.38 AE 0.06 gakidney vs 1.66 AE 0.06 gakidney (P`0.05). While total renal DNA was 16% greater in the obese: 18.2 AE 0.5 mgakidney vs 21.1 AE 0.6 mgakidney (P`0.05), no signi®cant difference existed when the DNA was expressed as mgamg protein.Fractional water content of the intact kidney declined with obesity: 78.7 AE 1.1% vs 76.0 AE 1.2% (P`0.05). Conversely, the hemoglobin content of the kidney at autopsy, an estimate of the unstressed vascular volume, increased with obesity: 55 AE 19 mgakidney vs 82 AE 25 mgakidney (P`0.05). By contrast, water content of renal parenchyma was constant: 80.8 AE 1.0% vs 80.9 AE 1.2% (cortex); 84.0 AE 0.8% vs 83.6% AE 2.0% (outer medulla); and 85.7 AE 0.8% vs 86.0 AE 2.1% (inner medulla). CONCLUSION: The renal growth associated with obesity was predominantly hyperplastic and was associated with a partial exclusion of¯uid from the renal sinus.

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