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Intravenous injection of synthetic mammalian gonadotrophin releasing hormone (GnRH; 5 µg per fish) induced a significant increase in androgen levels in the circulation of male dogfish within 4 h. Female dogfish injected with 10 µg GnRH in November showed significantly raised plasma levels of androgens and oestradiol-17β after 90 min as well as an increase in the rate of oviposition. Twice this dose of GnRH failed to produce significant increases in plasma levels of oestradiol in female dogfish in February and the androgen response in these animals was less than that seen after injection of GnRH in November.

Injection of an acid extract of dogfish hypothalamus significantly increased plasma levels of androgen and oestradiol in female fish when they were compared with control fish injected with extracts of dogfish cerebellum and the latency of response was similar to that encountered when GnRH was injected. It was concluded that the dogfish hypothalamus contains a factor capable of increasing plasma sex steroid levels in the same species and which is similar in its effects to mammalian GnRH.