The effect of underfeeding during autumn lactation, oestrogen priming and stage of the oestrous cycle on the release of LH in response to GnRH was studied in Merino ewes. The basal LH level prior to GnRH administration was not influenced by the treatments applied. Oestrogen priming significantly increased the peak LH level, both on days 3 (P < 0,001) and l5 (P < 0,05) of the oestrous cycle. in comparison, the total LH release showed a less consistent response to oestrogen priming. Ewes treated when pituitary stores of LH were expected to be high (day l5) released more LH (peak LH and total LH) than when treated early in the cycle (day 3) . Underfeeding reduced the sensitivity of the ewes to GnRH since a lower peak LH level was observed in such animals when compared to ewes which maintained their bodymass during lactation day l5; P ˂0,001).