Seventeen bulls were used for a 12-month survey of semen quality and for the estimation of plasma progesterone and testosterone and semen testosterone concentrations. Sperm output showed two minima, in mid-winter and late summer, and the percentage of abnormal sperm was highest and their ability to survive freezing was lowest at the summer minimum. Plasma progesterone concentration was negatively correlated with the total ejaculate content of testosterone, and positively with the local average maximum daily temperature. Temperature showed a quadratic relationship with the percentage of abnormal sperm ejaculated one month later, with the minimum percentage occurring at 14.5 degrees C. Temperature also showed a quadratic relationship with the numbers of sperm in semen ejaculated two months later, with the maximum number occurring at 17 degrees C. These relationships may reflect the impaired or enhanced survival of the temperature sensitive meiotic prophase and alterations in the output of testosterone and progesterone by the testis.