Chorionic gonadotropin (CG) in implantation sites from days 5-12 and placentae from days 13-19 of pregnancy in the mouse was determined by utilizing a cross-reaction with human CG in a radioimmunoassay. Significant amounts of CG could be detected throughout the period of gestation investigated. The amount of CG per implantation site increased steadily from day 5 to a peak on day 11. In the 2nd part of pregnancy a 2nd peak was observed with the highest amount of CG per placenta found on day 16. No gonadotrophic activity could be detected in blastocysts flushed from the uteri on day 5 of pregnancy or in uteri of non-pregnant mice.