The possibility of detecting lipid peroxidation histochemically by means of the 3-hydroxy-2-naphthoic acid/Fast Blue B (NAH-FBB) reaction was evaluated microspectrophotometrically. The procedure was modified in order to prevent exposure of tissue sections to lipid solvents. In fresh rat or mouse liver cryostat sections exposed in vitro to various prooxidant conditions (NADPH-Fe2+, NADPH-ADP/Fe3+, BrCCl3-NADPH), a close correlation was found between the intensity of the NAH-FBB (blue-violet) stain and the amount of malondialdehyde - taken as biochemical index of lipid peroxidation - released in the incubation medium. Stain intensities obtained with NAH-FBB reaction were several fold higher than those obtainable with direct Schiff reaction - a previously used procedure - and better parallelled in time the appearance of lipid peroxidation in tissue. In particular, by means of selective delipidation it was observed that NAH-FBB reaction is remarkably more efficient than Schiff reaction in detecting protein and phospholipid-associated lipid peroxidation-derived carbonyl functions. The ability of the NAH-FBB reaction to reveal lipid peroxidation occurring in tissues in vivo was verified with animals intoxicated with prooxidant toxins, i.e. the haloalkanes bromotrichloromethane and carbon tetrachloride, and the glutathione-depleting agent bromobenzene. In livers from haloalkane-treated rats, NAH-FBB positivity provided with the specific absorption spectrum was observed in centrolobular regions. In bromobenzene-poisoned mice, NAH-FBB positivity with specific absorption was found - besides the liver - also in kidney (tubular epithelium) and lung (bronchiolar epithelium). The use of the NAH-FBB reaction is therefore suggested for the discrimination of cell types undergoing lipid peroxidation in vivo.