Total, free and bound sialic acid concentrations were determined in sperm-free luminal fluid removed by micropuncture from different regions of the rat and hamster epididymides. In the rat, total sialic acid concentrations (mean +/- s.e.m.) in the proximal caput, the mid corpus and the proximal cauda were, respectively, 25.7 +/- 1.3, 23.9 +/- 1.7 and 28.8 +/- 1.9 mM compared to 4.4 +/- 0.1 mM in blood plasma. In the hamster, total sialic acid concentrations in the distal corpus, the proximal cauda and the distal cauda were, respectively, 32.9 +/- 3.8, 26.4 +/- 1.4 and 26.6 +/- 3.2 mM compared to 4.7 +/- 0.5 mM in blood plasma. Free sialic acid accounted for approximately 70-80% of total sialic acid present in the epididymal plasma of the rat. Similarly, 82% of sialic acid in the rat blood plasma was in free form. The levels of free and bound sialic acid were not changed in different regions of the rat epididymis. Unilateral ligation of the rat efferent ducts had no effect on total, free or bound sialic acid concentrations in all regions except in the proximal cauda in which a transient increase (P less than 0.01) in free sialic acid was observed on Day-3 after EDL.