There was usually an increase in the manometrically determined O2 uptake of rabbit spermatozoa which has previously been subjected to an incubation in the uterus of the doe, and also an increase in aerobic glycolytic activity as estimated by glucose utilization and lactate accumulation. Freshly ejaculated rabbit spermatozoa produced aerobically about as much labelled CO2 from [8-^Cjglucose as from [l-14c]glucose, whereas spermatozoa incubated in utero oxidized [1-14C]glucose at a rate distinctly above that shown by fresh spermatozoa. The time required for the development of these metabolic changes was similar to that required for the capacitation of rabbit spermatozoa. Rabbit spermatozoa maintained in vitro for 2 hr. or more at 37[degree]C suffered a marked decline in metabolism. The yeild of 14CO2 from [1-14C]glucose, however, declined less than that from [6-14c]glucose.