In the horse, pronounced changes in fertility occur annually in response to photoperiod. However, the mechanisms regulating gonadotrophin synthesis and release in this species remain unclear. Here, we investigated the expression of gonadotrophin subunits and GnRH receptor (GnRH-R) mRNA in the pituitary glands of Thoroughbred horses during the breeding (BS) and non-breeding (NBS) season. Seasonal effects on the prevalence of gonadotrophs in the pars distalis were also examined. GnRH-R and common alpha-, LHbeta- and FSHbeta-subunit mRNA contents were determined by Northern analysis and the prevalence of LH-gonadotrophs assessed by immunohistochemistry in pituitaries from sexually active females (mares) in the BS, and sexually inactive mares in the NBS. These variables were then measured in castrated male horses (geldings). In mares, pituitary content of FSHbeta mRNA was significantly higher in the NBS (P<0.01). Conversely, the content of common alpha-subunit mRNA was significantly higher during the BS (P<0.05). In contrast, GnRH-R and LHbeta mRNA abundance were unaffected by season. Interestingly, whereas no seasonal effects were apparent on the number of LH-gonadotrophs/field, the proportion of LH cells (in relation to all other cells) was higher in BS than NBS animals (P<0.05); this resulted from an increased number of non-gonadotroph cells during the NBS (P<0.05). In geldings, no significant seasonal effects were detected for any of the variables investigated (P>0.05). These results reveal robust seasonal effects on common alpha-subunit and FSHbeta gene expression in the pituitary of the mare, in the absence of detectable changes in the content of LHbeta or GnRH-R mRNA.