In the absence of a glycolyzable substrate, ATP levels and the motility of human sperm decline rapidly. This decline is not appreciably affected by the addition of succinate which is oxidized at a high rate or by pyruvate which is oxidized more slowly. If glucose is present initially, ATP levels are maintained constant over a 3-hr test period. Moreover, the addition of glucose to sperm suspensions, previously given oxidizable substrates only, markedly increases motility and ATP levels. Possible causes for this apparent inefficiency of human sperm oxidative metabolism and for the low rates of oxidation of pyridine-nucleotide-linked substrates are discussed.