Infusion of atrial natriuretic peptide (ANP) increases the glomerular filtration rate (GFR), and ANP is released from cardiac myocytes in response to extracellular fluid volume expansion. Since diabetes mellitus is associated with glomerular hyperfiltration and volume expansion, we investigated the relationship between ANP and GFR in diabetic rats given insulin to achieve stable moderate hyperglycemia or normoglycemia. At 2 wk after induction of diabetes, moderately hyperglycemic diabetic rats exhibited elevations of plasma ANP levels averaging 281 +/- 28 pg/ml vs. 158 +/- 15 pg/ml in normoglycemic diabetic rats. In hyperglycemic rats, the GFR was also elevated on average to 2.24 +/- 0.28 ml/min as compared with 1.71 +/- 0.13 ml/min in normoglycemic diabetic rats. To test further the relationship between ANP and GFR in diabetes, moderately hyperglycemic diabetic rats were infused either with a specific ANP antiserum or with nonimmune serum. The infusion of specific ANP antiserum uniformly reduced the GFR on average from 2.38 +/- 0.1 ml/min to 1.60 +/- 0.1 ml/min, whereas the infusion of nonimmune serum was without effect. It is concluded that elevated endogenous ANP levels contribute to the hyperfiltration observed in early diabetes in the rat.