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Western forms of the spotted skunk, Spilogale putorius latifrons, breed in late September. The fertilized oocytes undergo cleavage and blastulation but further development is greatly retarded and implantation is deferred for 180 to 200 days. Nidation occurs in mid-April and parturition takes place in mid-May (Mead, 1968). The hormonal control of delayed implantation in this species and other members of the family Mustelidae has long been a perplexing prob- lem. Numerous investigators have suggested that delayed implantation might be the result of inadequate progesterone secretion; however, all attempts to induce nidation by injections of progesterone, have, thus far, failed (Hansson, 1947; Hammond, 1951; Canivenc & Laffargue, 1958; Cochrane & Shackelford, 1962; Wright, 1963). It has been conjectured that the temporary inhibition of nidation results from a negative feedback system involving increased oestro- gen and reduced gonadotrophin secretion or that the high concentrations of oestrogen suppress the effects of progesterone (Hansson, 1947; Canivenc, 1966). Cochrane & Meyer (1957) observed that progesterone is essential for maintain- ing viable blastocysts and that oestrogen is required to induce nidation in ovariectomized rats. Shelesnyak & Kraicer (1963) reported that anoestrogen surge' which sensitizes the uterus and makes it capable of responding to deci- dual-inducing stimuli, is required before nidation can occur in the rat. This has led to the belief that anoestrogen surge' may also occur before nidation in mustelids. However, all attempts to induce nidation by injections of oestrone or oestradiol in intact or ovariectomized members of the latter family have also failed. In the light of the repeated failure of other investigators to induce nidation by injections of steroid hormones, we decided to determine the normal plasma levels of three ovarian steroids throughout the reproductive cycle. The results of the progesterone determination have been reported previously (Mead & Eik-Nes, 1969). The levels of free oestrone and oestradiol-17/? in pooled peripheral plasma samples (15 to 30 ml), obtained from spotted skunks, Spilogaleputorius latifrons, as previously described (Mead & Eik-Nes, 1969), were determined by the method of Mead, Haltmeyer & Eik-Nes (unpublished data) using gas-phase chromato- graphy and electron capture detection. The lowest concentration of either oes¬ trogen which can be determined by this method is about 1 nanogram (ng) per

OESTROGEN LEVELS IN PERIPHERAL BLOOD PLASMA OF THE SPOTTED SKUNK