Investigations of plasma gonadotrophin concentrations in intact ewes have shown that, in general, LH concentrations are low and fairly stable whereas FSH levels, which are much higher, may fluctuate from 15 ng to over 100 ng FSH/ml (Salamonsen et al., 1973). However, this apparent independence of the tonic levels of plasma LH and FSH is replaced by a close association of surges of both hormones at oestrus or when provoked by oestrogen, LH-RH, or progesterone-implant removal (Reeves et al.In marked contrast, plasma gonadotrophin concentrations in the mature ram show episodic fluctuations in plasma LH, which are not correlated with changes in FSH levels (Katongole, Naftolin & Short, 1974;Lee et al., 1976a). This difference between ewes and rams is also evident in the response to LH-RH; the mature ram shows an increase in plasma LH but very little or no increase in plasma FSH concentrations (Hopkinson, Pant & Fitzpatrick, 1974;Lee et al., 1976b).Following observations on the effect of oestrogen on plasma LH concentrations in freemartin sheep and heifers (Short, 1974;Saba, Symons, Cunningham & Boarer, 1976) which suggested that responses could be of the male or female type, it was of interest to determine the pattern of plasma hormone levels in freemartin sheep. This paper presents data on some plasma hormones in a mas¬ culinized freemartin ewe, its male sibling, three normal rams, and four wethers. MethodsAll the sheep used were Dorset Horns; the three rams, freemartin ewe and its male sibling were 12 months and the wethers 18 months old when the studies were conducted. The freemartin and its male sibling were the surviving pair of triplets : at birth fetal distribution was found to be freemartin and dead male in the left uterine horn and live male sibling in the right horn. The freemartin had palpable testes in the inguinal region, displayed aggressive male behaviour and had 60% XY cells in the blood. The male sibling was apparently normal. The sheep were housed indoors and jugular vein samples were taken every 2 h for 24 h from the rams, freemartin and male sibling (in March), and at 1-h intervals for 8 h from the wethers (in May).Plasma FSH was determined by the solid-phase assay as described by Cunningham & Hebert (1973). Plasma LH was assayed by a solid-phase method utilizing PVC haemagglutination trays coated with antiserum (Boarer, Morris & Mackintosh, 1977): the intra-assay variation was 9-6% and the sensitivity was 0-5-1 ng LH/ml plasma. All plasma samples were run in the same assay. FSH and LH concentrations are expressed as ng of NIH-FSH-S4 and NIH-LH-S17 respectively/ml. Plasma samples (100-500 µ ) were extracted with 5 ml light petroleum ether (b. pt. 40-60°C) for progesterone determinations or 5 ml diethyl ether for testosterone, androstenedione and free oestrogens, and the steroids in the extracts were assayed by the charcoal-gelatin disk method (Saba, 1976). Results and discussionWethers. The plasma concentrations of LH and FSH fluctuated irregularly during the 8-h observation period, but there was n...