In view of significant species-specific sequence differences between human and rat placental hepatocyte growth factor (HGF)/scatter factor (SF), the rat placental HGF/SF (rpSF) was purified, and its properties compared with human placental HGF/SF (hpSF). Like hpSF, rpSF scattered Madin-Darby canine kidney cells at 1-2 ng/ml and is composed of two subunits of 60 kDa and 30 kDa. Higher amounts (> 50%) of uncleaved 90-kDa form was present in the HGF/SF preparations from both human and rat placentas. Rat placental SF reacts with antibodies raised against hpSF in rabbits and chickens. The SF activity when expressed per gram rat placental tissue rises rapidly up to 9 days and then levels off. When expressed per milligram tissue protein it also increases rapidly up to 9 days and then declines. The expression of HGF/SF mRNA during development parallels that of HGF/SF activity. The specific activity of HGF/SF receptor (c-met) mRNA also appears to peak at 6 days. These findings suggest that (i) in spite of significant (> 10%) sequence differences between rpSF and hpSF, they exhibit similar structural, biologic, and immunologic characteristics and (ii) HGF/SF and its receptor are expressed in high amounts on Day 6 and then decline in developing placenta.