The lipid composition of hamster epididymal spermatozoa was examined. Caput epididymal spermatozoa were isolated by Percoll density gradient centrifugation without contamination by other cells and they had a specific gravity of 1.10-1.12 g cm-3. Caput and cauda epididymal spermatozoa showed little difference in the amounts of total fatty acid and total sterol. However, sterol composition changed markedly during the transit of spermatozoa through the epididymis: the amount of cholesterol decreased, while the amount of desmosterol and cholesta-7,24-dien-3 beta-ol increased. No significant change in fatty acid composition was observed during the transit, although there was a tendency for an increase in chain length. Both 22:5 and 22:6 represented high percentages in fatty acids of hamster spermatozoa. Some difference in lipids was detected between the upper fraction (1.04 g cm-3) and the lower fraction (1.10 g cm-3) obtained by the density gradient centrifugation of cauda epididymal spermatozoa. Total fatty acid content of the upper fraction was 1.4-fold higher than that of the lower fraction, and the percentage of 18:0 was lower in the latter fraction with a higher percentage of 18:2. The total sterol:total phospholipid ratio in hamster cauda epididymal spermatozoa was 0.21.