Progesterone patterns in the peripheral plasma of mares have recently been reported by Smith, Bassett & Williams (1970), Stabenfeldt, Hughes & Evans (1971), and by Plotka, Witherspoon & Foley (1972). In the research reported here, progesterone levels were measured in the peripheral circulation of pony mares.
Mature pony mares of mixed breeding were checked daily for oestrus by teasing with a vigorous stallion. Blood samples were collected daily from indwelling jugular vein cannulae. After extraction of progesterone from 1- or 2-ml plasma samples with diethyl ether, the ether extract was partially concentrated by evaporation under nitrogen, and transferred to pre-coated thinlayer chromatography plates (Machery-Nagel). Progesterone was separated from other ether-soluble plasma components by one-dimensional chromatography in ether : benzene (2:1). Scraping the thin-layer plates with a razor blade to remove progesterone yielded a tight cylinder of silica gel small enough to be placed into the barrel of a Pasteur pipette, the