The activity of ornithine decarboxylase (ODC) in the rat anterior pituitary gland varies during the oestrous cycle, with a rise in activity seen at pro-oestrus. This enzyme, which is rate-limiting for the synthesis of the polyamines, can be specifically and irreversibly blocked by alpha-difluoromethylornithine (DFMO). A previous study showed that when this drug was administered to rats in vivo on the afternoon of pro-oestrus, it suppressed the normal surge in plasma prolactin levels that occurred later that day. The effect of DFMO was associated with reduced levels of putrescine in the anterior pituitary gland, suggesting that ODC activity in the lactotroph might be involved in the prolactin surge. We have examined the effects of DFMO on the secretion of prolactin from anterior pituitary cells, isolated either from male rats or from females at different stages of the oestrous cycle. The drug was found to reduce prolactin secretion stimulated by thyrotrophin-releasing hormone (TRH), but only in cells isolated from pro-oestrous animals and only for 2 days after cell isolation. Basal secretion was unaffected by DFMO. The results imply that ODC is important for TRH-stimulated prolactin secretion at pro-oestrus, and it is specific for pro-oestrus. The prolactin surge could therefore be influenced by this ODC-dependent effect of TRH: The pro-oestrous-specific response to TRH may be a consequence of the increased ODC activity seen at this time. Alternatively, the increased ODC activity could be a consequence of coupling to TRH receptors, which are known to increase in number at pro-oestrus.