Two cows with synchronised oestrous cycles were housed with a Friesian bull. During the midcycle period of the cows, one cow was given an intradermal injection of adrenaline in the perineal region. This caused local sweat gland discharge. The other cow was treated with water. The number and types of bull-to-cow behaviours were recorded before, during and after the day of treatment. This was repeated at three-weekly intervals for a total of four times. On the day of treatment the bull directed a greater proportion of olfactory behaviours towards the adrenaline-treated cow than the other cow (P less than 0.05). There was no difference between the cows before or after the day of treatment. The above experiments were repeated with a Hereford bull who showed the same response as the Friesian for the first two but not the last two experiments. The increased proportion of bull olfactory behaviours elicited by an increased perineal skin gland discharge, adds support to our hypothesis that the perineal skin glands are the source of an oestrous pheromone in the cow.