Microdetermination methods were used to determine the activities of hexokinase in human and mouse oocytes, human spermatozoa, and other somatic cells. Human oocytes with intact germinal vesicle obtained from the growing follicle were freeze dried and weighed (mean +/- SD = 243 +/- 34 ng dry weight). Hexokinase activity in single oocytes was 17.9 +/- 3.6 pmol of reduced nicotinamide-adenine dinucleotide phosphate formed/oocyte/hr at 20 degrees C. Specific activity was estimated to be 104 +/- 30 nmol/mg protein/hr, which was remarkably lower than that in human spermatozoa (3570 +/- 550 nmol/mg protein/hr) and other somatic cells such as endometrium, brain, liver, and kidney. Mouse follicular oocytes and early embryos before the two-cell stage also had low hexokinase activities, but morulae and blastocysts had increasingly high activities; the former cannot use glucose as an energy source, whereas the latter can. These results suggest that human immature oocytes, as well as mouse oocytes, depend on pyruvate instead of glucose as a major energy source.