A new progestational steroid, the acetophenone derivative of 16α,17α;- dihydroxyprogesterone, was compared with several progestogens for masculinizing effects on the offspring of rats treated during pregnancy. The results were similar to those obtained with progesterone and 17α-hydroxyprogesterone caproate: this new steroid did not alter the normal male:female sex ratio and did not virilize the internal or external sex structures of the newborn. The present results confirm those of others in finding that 6α-methyl-17α-hydroxyprogesterone acetate, 17α-ethinyl-19-nortestosterone and 17aethinyl- 19-nortestosterone acetate increased the number of male and intersex offspring, as determined by ano-genital measurements and masculinization of internal and external sex organs.