For two weeks prior to the start of the breeding season, the suckling behaviour of beef calves, separated from their dams for 12 h out of 24, was investigated. Fifty-six per cent of the 44 cows which were separated from their calves from 18:00 to 06:00 (day-sucklers) exhibited oestrus within 30 days of the start of the breeding season, compared to 22% of the group separated from 06:00 to 18:00 (P <0.01). It was concluded that not only the suppression of suckling may be involved in the onset of ovarian activity. The specific time period during the 24 h when suckling is prohibited may also play a role.