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Three stallions were bled each hour for 25 h at 28-day intervals throughout 1 year. Testosterone levels were pulsatile. Pulse frequency and pulse amplitude were higher in the summer months than at other times (P<0·01). The number of testosterone pulses also varied throughout the day, with the greatest frequency occurring in the afternoon (14.00–17.00 h) and at night (22.00–01.00 h). Mean testosterone levels were highest in the summer (P<0·01) but showed a secondary, smaller increase in the autumn. Semen characteristics were assessed from measurements of 222 stallions. Semen volume was greatest in the summer (P<0·01) but both the concentration (P<0·01) and the total number of spermatozoa per ejaculate (P<0·05) were highest in the autumn. These results show that the highest testosterone concentrations in peripheral plasma are not necessarily associated temporally with optimum semen quality.