THYROXINE CONCENTRATIONS ARE ELEVATED IN MARES WHICH CONTINUE TO EXHIBIT ESTROUS CYCLES DURING THE NONBREEDING SEASON

In two successive years, the pattern of secretion of thyroxine was com pared in seasonally anestrous mares and a subpopulation of mares that exhibited estrous cycles during the nonbreeding season. Blood samples were collected two or three times per week between October and April f rom a total of 15 mares that exhibited estrous cycles during the nonbr eeding season and 21 anestrous mares. Monthly mean concentrations of t hyroxine during the period from October through April were greater (p< 0.05) in cycling versus anestrous mares. However, it remains to be det ermined whether continuation of estrous cycles during the nonbreeding season and elevated concentrations of thyroxine are causally related.