Many mammalian species utilise day-length (photoperiod) to adapt their physiology to seasonal changes in environmental conditions, via secretion of pineal melatonin. Photoperiodic regulation of prolactin secretion is believed to occur via melatonin-mediated changes in the secretion of a putative prolactin secretagogue, tuberalin, from the pituitary pars tuberalis. Despite the in vivo and in vitro evidence in support of this intra-pituitary signalling mechanism, the identity of tuberalin has yet to be elucidated. This paper reviews recent advances in the characterisation of tuberalin and the regulation of its secretion. Furthermore, the hypothesis that pituitary lactotroph cells display heterogeneity in their response to changing photoperiod and tuberalin secretion is examined.