Oxygen consumption and percent motile spermatozoa were determined for semen samples from healthy male monkeys Macaca fascicularis. Methyl mercury was added to the samples, in the oxygen measurement chamber, at a concentration of 9 p.p.m. for 15 min. and then increased to 15 p.p.m. for 15-30 min. Oxygen consumption and percent motility were determined during each period. Methyl mercury addition resulted in decreased sperm motility, but we did not detect any inhibition in the rate of oxygen consumption accompanying the decreased motility. On the contrary, the rate of oxygen consumption increased at 15 p.p.m. within 15 min., while the sperm motility was close to zero. Antimycin inhibited the increased rate of oxygen consumption demonstrating the mitochondrial source of this increased rate. Oligomycin also inhibited the increased rate of oxygen consumption due to methyl mercury, thus excluding the possibility of uncoupling of mitochondrial oxidative phosphorylation. Interference with mitochondrial energy production does not seem to be the primary mechanism of methyl mercury-induced decreased spermatozoal motility. Methyl mercury interference with the dynein/microtubule sliding assembly now seems to us to be a more tenable hypothesis.