In many mammals, including man, lactation is associated with a delay in return of fertility. The pattern of suckling activity, particularly suckling frequency, is known to be important in determining the length of this anovulatory phase1,2. In a seasonally breeding ungulate such as red deer, lactation coincides with a photoperiodically controlled period of seasonal anoestrus3. Nevertheless, lactation may still exert an important effect on the duration of infertility as lactating red deer hinds on poor quality hill pasture resume oestrus at a later date than non-lactating hinds and frequently fail to ovulate in the year following the birth of a calf 4,5. It has been proposed that this reproductive failure is due to the effect of both lactation and poor planes of nutrition on maternal body condition6-8. As a result of our recent studies on the influence of plane of nutrition on milk yield and suckling behaviour in red deer we now propose an alternative hypothesis. We suggest that the principal influence of low planes of nutrition is to increase the suckling frequency of the calf in response to a decrease in availability of milk. It is this increase in suckling frequency, and perhaps the associated increase in plasma levels of prolactin, which is the major determinant of the reproductive failure and is due to the influence of plane of nutrition on the production of milk by the mother rather than on maternal body condition. This hypothesis may have important implications for our understanding of the role of nutrition in determining the length of lactational infertility in many species including man. © 1983 Nature Publishing Group.