To determine if changes in dietary protein intake alter renal excretion of small molecular weight proteins in passive Heymann nephritis, 21 rats with passive Heymann nephritis were fed 8.5% protein for 12 days after injection with antiserum. Dietary protein intake was then increased to 40% in 10 rats (LP-HP) while 11 rats remained on 8.5% protein (LP-LP). Lysozymuria (UlysV) increased from 66.5 +/- 31.0 mcg/day to 457.5 +/- 98.0 mcg/day (P less than 0.001) after five days in LP-HP, but was unchanged in LP-LP. Albuminuria (UalbV) increased only in LP-HP, from 168 +/- 23 mg/day to 447 +/- 45 mg/day (P less than 0.001). Urinary lysozyme excretion correlated with UalbV (r = 0.737, P less than 0.001), and changes in UlysV correlated with changes in UalbV (r = 0.657, P less than 0.01). To determine whether the increase in UlysV was the direct effect of the change in diet, enalapril 40 mg/kg/day was administered to prevent the increase in UalbV that occurs when these rats are fed a high protein diet. Twelve rats were fed 8.5% (LP) and 10 were fed 40% protein (HP) from the time of injection with antiserum. Six LP (LPE) and five HP (HPE) received enalapril. UlysV was 873 +/- 391 mcg/day in HP and nearly undetectable in the other three groups. UalbV was significantly greater in HP (368 +/- 60 mg/day) compared to the other three groups (114 +/- 16 in LP, 136 +/- 44 in HPE, 95 +/- 21 in LPE.(ABSTRACT TRUNCATED AT 250 WORDS)