17β-Oestradiol, progesterone, testosterone, PMS and HCG were tested for their possible effects on the metabolism of washed bull spermatozoa when incubated in Krebs-Ringer phosphate medium at 37° C. Steroid hormones suppressed the oxygen uptake of the spermatozoa, and this was particularly evident with progesterone and testosterone the addition of which led also to a rise in substrate (glucose and pyruvate) utilization and in lactate production. Steroid hormones (1 mg/4 ml suspension) produced their effects when they were added to the medium as crystals or after they had been dissolved in propylene glycol, but more effectively in the latter case. The anaerobic glycolysis of spermatozoa, however, was not influenced by steroid hormones. PMS and HCG at the doses used (20 IU/4 ml suspension) did not affect the metabolism of spermatozoa. The results are discussed in relation to the possibility that steroid hormones may play a direct part in regulating the metabolic pattern of spermatozoa, which may be, in turn, associated with a shift from the epididymal to the ejaculated and then to the capacitated pattern.