The present investigation has been undertaken to elucidate the functional role of ovarian steroids on the mechanism of oviduct differentiation during early embryonic development in rat. The activity of alkaline phosphatase (ALPase) was measured in the oviduct tissue under different steroids treatment regime on day 9 pregnancy. The ALPase activity of the oviduct of pseudopregnant rat was compared with that of normal pregnant rat. The results of day 9 pregnancy rat oviduct clearly demonstrated that $17{eta}-estradiol$ and progesterone were effective in pseudopregnant rat oviduct. In the ovary intact group the ALPase activity was similar in both of normal and pseudopregnant oviduct, but in the $17{eta}-estradiol$ treated group the ALPase activity in normal pregnancy was significantly higher than that in pseudopregnancy. The effect of estradiol on the normal pregnant rat oviduct was apparently found on day 3 and day 9 pregnancy. This study, therefore, clearly demonstrates that $17{eta}-estradiol$ is much potent in oviduct tissue differentiation. It is suggested that absence of $17{eta}-estradiol$ effect on pseudopregnant rat oviduct is due to there is no embryo passing througth the oviduct.