AbstractThe role of endogenous opioids and the dopaminergic system on the inhibition of luteinizing hormone (LH) secretion during early and late anoestrus, together with its modulation by the plane of nutrition were investigated in ewes with a short anoestrous season. In early anoestrus (22 March; day 0), two groups of ovariectomized, oestradiol-treated adult Rasa Aragonesa ewes, maintained under natural photoperiod at 41°N, were given enough food to provide 1·4 × (high; H; no. = 6) or 0·5 × (low; L; no. = 6) energy requirements for maintenance. The effects of administration of the opiate receptor antagonist naloxone (1 mg/kg at four 1-h intervals) (day 15) and of the dopaminergic2 receptor antagonist pimozide (0·08 mg/kg) (day 21) on LH secretion were assessed. A second experiment was carried out in late anoestrus (21 June) using the same protocol. A significant increase in LH pulse frequency after naloxone treatment for both H and L groups was detected in late anoestrus. Number ofLH pulses after naloxone injections in early anoestrus also increased in H (P < 0·05) and L ewes (P = 0·08). The effect of pimozide injection on mean LH pulse frequency was greater in early than in late anoestrus, especially in ewes receiving a high plane of nutrition (P < 0·05 and P = 0·07 for H and L ewes, respectively in April and P = 0·07 for H ewes in July). A significant increase of LH pulse amplitude was also detected in early anoestrus in H ewes (P < 0·01). These results provide evidence that endogenous opioid mechanisms are involved in the inhibition ofLH pulsatile release both in early and late anoestrus in ewes with a short seasonal anoestrus. The ability of pimozide to increase LH pulse frequency in early anoestrus could be enhanced by a high plane of nutrition in the breed studied.