Placental Na+, K+ -ATPase activities were measured in connection with the active transport of amino acid, intra-uterine growth retardation (IUGR) or small for date infants (SFD) in salt-loaded pregnant SHRSP. 2. All parameters such as fetal weight, placental weight and Na+, K+ -ATPase activity in the placental tissue were significantly depressed in SHRSP, especially in the salt-loaded SHRSP compared with WKY. 3. Besides a decrease of blood flow, a decline of Na+, K+ -ATPase activity associated with an active transport of amino acid in the placenta would cause IUGR and SFD in the salt-loaded pregnant SHRSP. 4. Pregnant SHRSP given water containing salt would be a good model for pregnancy induced hypertension in humans.