The relationships between scrotal circumference and live weight and sperm output were examined in 3 samples of bulls selected for use in AI. In Study 1 the linear correlation between live weight and scrotal circumference in 418 British Friesian bulls at about 400 days of age was +0.28, and the circumference of the scrotum was related to the live weight of the bull by +0.179 +/- 0.060 mm/kg live weight. At 400 days of age the mean (+/- s.e.) live weight and scrotal circumference of 22 British Friesian bulls in Study 2 was 412 +/- 7 kg and 332 +/- 4 mm respectively. Rates of growth were 1.11 +/- 0.023 kg/day and 0.426 +/- 0.023 mm/day. The correlation (+0.42) between the size of the scrotum and the no. of spermatozoa/ejaculate was not significant. British Friesian (average age 90 years) and Hereford (average age 6.5 years) bulls were examined in Study 3. For the 25 Herefords, the correlation between the number of usable straws over a 6-month period and the scrotal circumference was +0.43, compared with +0.21 among the 28 British Friesians. It is concluded that scrotal circumference is unlikely to be an accurate predictor of sperm output in AI bulls.