The relative effectiveness of 20[alpha]-hydroxypregn-4-en-3-one and progesterone on induction of es-trous behavior was tested in estrogen-primed spayed female rats. Two or more lordosis responses within the first 7 hr after progestin injection were considered to be a positive indication of behavioral estrus. Results obtained on 36 rats during a total of 171 tests showed the hydroxylated steroid to be more effective in the presence of estrogen than estrogen alone, but less effective than estrogen-progesterone as an inducer of mating behavior.