Glucose has been reported to be beneficial to human sperm for optimal capacitation and fertilization, although it is unclear whether glucose is required for providing extra metabolic energy through glycolysis, or for generating some other metabolic product. In this study, the effects of sugars on human sperm capacitation, motility, and energy production were investigated. The glucose concentration that supported the greatest number of acrosome reactions was 5.56 mmol L(-1). Compared with incubations with no added sugar, this concentration of glucose, fructose, mannose, or galactose appeared to slightly increase the number of acrosome reactions occurring after 18 hours of capacitation, or following induction by 2 micromol A23187 + 3.6 mmol pentoxifylline L 1, but only glucose had a statistically significant effect. Glucose supported increased penetration of zona-free hamster oocytes, but its advantage was not statistically significant. The addition of 5.56 mmol glucose or fructose L(-1) to sugar-free medium immediately increased the adenosine triphosphate (ATP) concentration and motility of sperm. These parameters were then stable for 3 hours, but declined markedly after 18 hours. In the absence of a glycolysable sugar, motility began to decline in the first hour and only 2% or 3% of sperm remained motile after 18 hours. Glucose or fructose was required to support hyperactivated motility. 2-Deoxyglucose was detrimental to the ATP concentration and motility of sperm, and supported fewer spontaneous or progesterone-stimulated acrosome reactions than were observed in the absence of a sugar. We conclude that glycolytic ATP production is required for vigorous motility and hyperactivation in human sperm. Other products of glucose metabolism are not essential to support capacitation, but they may have a small, enhancing effect.