A direct time-resolved fluorescent immunoassay (TR-FIA) system was developed and evaluated for measuring progesterone (P) concentrations in deer plasma. The minimum detectable level of this assay system was 0.25 ng/ml and ED 50 value of standard curve was 2.4 ng/ml. Intra- and inter-assay coefficients of variation were 8.8% and 15.7%, respectively. The direct TR-FIA revealed that the plasma P concentrations in sika deer expressed a diurnal pattern depending on their reproductive status. Results in the present study indicate that the TR-FIA system, that is specifically designed for humans if successfully utilized with slight modification, can be utilized for direct assaying of P in deer plasma. The plasma P profile in the sika deer appears to be consistent to that of other temperate deer expressing seasonal breeding characteristics.