In the Milan hypertensive rats (MHS) glomerulosclerosis is less evident than in the normotensive strain (MNS). To clarify whether this pattern is due to a 'protective effect' of increased afferent arteriolar tone or to a different mechanism, we studied 12 first-generation hybrids (F1), 4 parental MHS and 4 parental MNS rats. Four-micrometer sections were stained with hematoxylin, Mallory's trichrome stain and periodic acid-Schiff reaction. The blood pressure of the F1 rats was only slightly higher than that of the MNS so that very probably renal vascular resistances were similar. The F1 rats had low proteinuria (23.3 +/- 2.7 mg/24 h) like the MHS (25.3 +/- 4.8), and few damaged glomeruli per section (18.5 +/- 1.2), again like the MHS (18.7 +/- 1.1). MNS had higher proteinuria (363.8 +/- 111.6; p less than 0.01 vs. MHS and F1) with a greater number of damaged glomeruli (51.4 +/- 4.5; p less than 0.01 vs. MHS and F1). The difference in afferent arteriolar resistance is not implicated in glomerulosclerosis.