The fibroblast growth factors (FGFs) are a family of conserved polypeptides known to regulate cell differentiation and proliferation. We have used avidin-biotin-enhanced indirect immunohistochemistry to localize FGF-1 and FGF-2 in the rat kidney. The most consistent specific immunostaining pattern is found in paraffin sections from kidneys perfusion-fixed with 4% paraformaldehyde in 0.1 M phosphate buffer. Intracellular immunoreactivity for FGF-1 and FGF-2 is co-localized in visceral (podocytes) and parietal (Bowman's capsule) glomerular epithelial cells, S3 segments of proximal tubules, distal tubules and collecting ducts in the cortex, and thick ascending limbs and collecting ducts in the medulla. Immunoreactivity is also observed within urothelium and the tunica adventitia of large blood vessels. No immunostaining is found in cortical S1 or S2 segments of proximal tubules, in frozen sections prepared from unfixed or 4% paraformaldehyde perfusion-fixed kidneys, or in paraffin sections from Bouin-fixed kidneys. Immersion fixation with 4% paraformaldehyde gives a similar staining pattern in paraffin sections to that achieved with perfusion fixation. However, in paraffin sections fixed with methyl Carnoy's fixative, immunoreactivity is primarily localized to the tunica media of blood vessels, with little tubular or glomerular immunostaining. Thus, variation in immunolocalization patterns for FGFs can be partially attributed to differences in fixative, preparative technique and antibody specificity.