discontinued injection site assessments are made on days 0 1 3 6 and 9 post treatment scores are based on a subjective scale of 0 3 zero means no swelling one means slight diameter swelling i e about 12 5 mm 2 means moderate diameter swelling i e about 12 5 to 25 mm and 3 means significant swelling i e more than 25 mm plasma concentrations of progesterone are measured with commercially available reagents diagnostic systems laboratories webster tex intra and interassay cv and assay sensitivities are 5 8 and 0 1 ng ml data are analyzed by the proc mixed procedure of sas sas institute inc cary n c single point variables are analyzed via a one way anova data from injection site scores are analyzed for effects of treatment time and treatment by time interactions with repeated measures when a significant f is detected p lt 0 05 the least significant difference lsd test is used to determine differences between groups within times each of the single injection altrenogest formulations are effective at extending p lt 0 05 the return to estrus and interovulatory interval in mares when compared to controls as shown in table 2 the altrenogest mp 500 formulation suppresses estrus and inhibits ovulation the longest p lt 0 05 the mean days of return to estrus is 32 8 u00b1 4 8 days compared to 3 9 u00b1 0 5 days in the control mares days to ovulation relative to administration of dinoprost tromethamine and altrenogest for the mp 500 formulation is 34 7 u00b1 3 7 days compared to 8 3 u00b1 1 2 days in control mares both doses of the liquid altrenogest la 150 formulation effectively extends the return to estrus following treatment 12 7 u00b1 0 4 days.