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National Institute for Research in Dairying, Shinfield, Reading, RG2 9AT

(Received 8 May 1975)

Recent work in ruminants has shown that the plasma concentration of prolactin is markedly influenced by daylength and season (Pelletier, 1973; Buttle, 1974; Hart, 1975a, b) but no information is available on how the level of growth hormone (GH) in the circulation may vary in ruminants throughout the year. The concentration of GH was therefore measured in the samples of plasma obtained from three intact male goats in which the seasonal variation in prolactin concentration had already been demonstrated. Details of the experimental animals, the weekly blood sampling routine and the results for prolactin have been described by Buttle (1974). The goats were fed ad libitum throughout the 14-month sampling period. Analysis for GH was carried out according to the method of Hart, Flux, Andrews & McNeilly (1975) using ovine GH (NIH-GH-S10) as the standard preparation.