Renal insulin-like growth factor (IGF)-I expression is enhanced in tissue that remains following removal of kidney mass. To characterize the expression of renal IGF-I after reduction of kidney mass by partial unilateral infarction, we measured levels of IGF-I extracted from noninfarcted (remnant) renal tissue that remained after one-half unilateral kidney infarction that was performed without (1/2NX) or with (1 1/2NX) contralateral nephrectomy. Levels of IGF-I extracted from remnant renal tissue after 1/2NX increased significantly, peaking on day 3 after renal mass reduction, and then returned toward baseline. Steady-state levels of IGF-I mRNA were also elevated on day 3, suggesting that the increase in IGF-I results from enhanced synthesis. A similar pattern of increased extracted IGF-I and elevated IGF-I mRNA occurred after 1 1/2NX. Levels of IGF-I extracted from remnant renal tissue 3 days after 1 1/2NX were not higher than levels extracted from remnant tissue 3 days after 1/2NX, and both were higher than levels of IGF-I extracted from contralateral kidneys 3 days after unilateral nephrectomy. Therefore, levels of IGF-I did not correlate with the extent of reduction of renal mass per se. We conclude that partial renal infarction provides a stimulus to enhance IGF-I expression. Increased renal IGF-I must be considered in the interpretation of findings originating from use of remnant kidney models of chronic renal failure.