Seminal plasma from fasting nondiabetic donors has an insulin concentration of 19 ± 3 μU/ml which is more than twice as great as the blood serum concentration of the same persons (7.5 ± 1.5). Addition of glucose to a suspension of washed human spermatozoa inhibits the oxygen uptake (Crabtree effect). This action is not only overcome by the later addition of insulin (50 μU/ml), but a significant increase in oxygen uptake is produced (from 10.5 to 33.4 nmoles/min/109 spermatozoa). Insulin increases hexose utilization markedly when glucose is used as substrate and also when fructose is used. Since lactate production shows a significant decrease while trapped 14CO2 increases, this increase in hexose utilization is mainly due to the stimulation of the pentose and Krebs cycle. By radiorespirometric techniques using selectively labelled substrates, it was found that insulin produces a 50% increase in the glucose utilized by the pentose cycle and also an increase in the pyruvate dehydrogenase activity. Finally, insulin has a double effect on the utilization of pyruvate: it increases its decarboxylation (increased pyruvate dehydrogenase activity) and, more importantly, it induces an almost 4–fold increase in 14CO2 production from pyruvate carbon–2. (Endocrinology92: 833, 1973)