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Since the discovery of LH-RH and its subsequent synthesis, the study of pituitary function in various animals at all stages of reproductive life has become possible. Schally et al. (1971) showed that administration of natural or synthetic LH-RH released LH and FSH from the pituitary gland in animals of various species. Mauer & Rippel (1972) have shown that synthetic LH-RH (referred to by them as Gn-RH) was active in mature heifers. In this study six mature dairy cows (4 Ayrshire, 2 Friesian) that were 7-8 years old were used. Their pituitary response, as measured by circulating LH levels, to 500 \g=m\gLH-RH given i.v. was determined on 2 consecutive days between 16 and 32 days before parturition. This dose of LH-RH corresponded to approximately 1 \g=m\g/kgbody weight. Preliminary experiments in other cows have shown that this dose consistently caused LH release. Catheters were inserted into the jugular veins the day before an experiment to facilitate administration of LH-RH and withdrawal of blood. LH-RH was given in 10 ml 0\m=.\9 % saline and blood samples (10 ml) were taken 10 min before, and at 10, 20, 30, 45, 60 and 90 min, and 2, 3, 4, 5 and 6 hr after injection. The six cows gave birth to 6 bull calves and 2 heifers. The mean (\m=+-\S.E.M.) weight of the calves at 24 hr was 45\m=.\2\m=+-\5\m=.\8 kg. The response of 6 of these calves (4 male, 2 female) to 50 \g=m\g LH-RH injected i.v. in 1 ml 0\m=.\9% saline (about 1 \g=m\g/kg body wt) was determined within 30 hr of birth. Blood samples (10 ml) were taken from the jugular vein using heparinized Vacutainers at the same intervals as for the mothers. Blood samples were centrifuged, plasma separated and stored deep-frozen until assayed for LH. This was measured by an established solid-phase radioimmunoassay system (Symons, Cunningham & Saba, 1974). The antiserum was raised in rabbits with bovine LH (NIH-B4) and was used at a final dilution of 1:8000; bovine LH (NIH-B8) was used for iodination and standards. Iodination

The LH response of pregnant cows and their calves to synthetic LH-RH