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Adenylate kinase activity and adenine nucleotides, lactate and pyruvate concentrations were measured by enzymatic techniques in recently ejaculated human spermatozoa and semi-nal plasma. Determinations were made on a group of six subjects after 3 days of sexual ab-stinence, and then, in the same subjects, in the last of four consecutive daily ejaculations. In spermatozoa, adenylate kinase activity was not modified by frequent ejaculations but ATP fell from 0.44 to 0.16 moles/mg DNA while AMP increased from 0.008 to 0.04 tmoles/mg DNA, and ADP did not change. When these data were used to calculate the energy balance, itwas found that allthe parameters studied showed a significant decrease. This finding may explain the diminution in cell concentration, motility and vitality observed in semen after repeated ejaculations, and also with the decrease in fertility of spermatozoa. From the meas-ured pyruvate and lactate concentrations we calculated a NAD/NADH ratio of 2800, which is about four times higher than that reportedly found in rat liver. This ratio, together with the high pyruvate kinase activity found, may help to explain the high aerobic glycolysis of human sperm cells. Adenine nucleotides are important deter