8 adult rats received estrin for 2 mos., the doses varying from 0.25 to 5 R.U. a day. Cyclic changes in sexual excitability were prevented by this treatment. Behavior during the inj. period showed a fairly constant low degree of sexual receptivity, rated 0-3 on a scale of 12. Untreated heat periods before and after estrin administration were characterized by peak ratings of 7-12 in most cases. Injns. did not produce lasting damage to the reproductive functions.