It is well established that high environmental temperature can have adverse effects on sperm concentration and the normality of sperm in the boar. Induced abnormalities include reduced motility, abnormal heads, proximal droplets, coiled and bent tails and abnormal acrosomes (Malmgren and Larsson, 1989). While all stages of spermatogenesis can be adversely affected, the primary spermatocytes are most vulnerable to these influences. Since spermatogenesis occurs over 45 days, any adverse effects of elevated ambient temperature can affect sperm quality for around 6 weeks. The maximum period of infertility appears around weeks 3 and 4 after heat stress. Reduced levels of testosterone and sometimes of LH following heat stress are implicated in these adverse effects. There is evidence that boars subjected to high constant temperature (30°C), which have become acclimatised to such, are affected less in terms of sperm output and quality (Cameron and Blackshaw, 1980) than when boars are subject to sudden major increases in temperature from fairly low levels (15 to 30 C) e.g. during the summer months (Antonyuk et al, 1983). There also appears to be large differences between boars in their ability to adapt to exposure to high environmental temperature by minimising temperature rise and avoiding adverse effects on semen quality (Cameron and Blackshaw, 1980). These workers found that boars prone to heat stress show an acute rise in body temperature in response to elevated environmental temperature and this sudden rise has a more adverse effect on semen quality than the length of exposure to the elevated temperature. There appears to be little information available on the reasons for such important between boar differences. This study was carried out to determine rectal temperature responses of boars to varying summer temperature in an intensive pig enterprise in Scotland and to attempt to determine some of the factors associated with ‘high’ and ‘low’ responding boars.