Plasma samples were obtained from seven bitches during 10 oestrous periods. Progestogen concentrations were determined in 182 samples by radioimmunoassay and by both quantitative and qualitative enzyme-linked immunoassay (ELISA) commercial kits. The concentrations determined by the quantitative ELISA were consistently higher than those measured by radioimmunoassay although the values were highly and significantly correlated. The qualitative ELISA readily distinguished between low and high progestogen concentrations, ie, less than 1.0 ng/ml and greater than 2.6 ng/ml measured by radioimmunoassay. Sequential evaluation with either kit, especially when combined with vaginal cytology, should be useful in determining the optimum time for mating.