A new class of modified progesterones with an additional ring in the 16 alpha , 17 alpha-position (pregna-D'-pentaranes) are described. Compounds containing 4- and 6-membered D'-ring (D'4- and D'6-pentaranes) were synthesized by the cycloaddition of acetylene or 1,3-butadiene, respectively, to the conjugated 16-double bond of 16-dehydro-20-keto steroids. The D'3-pentarane was prepared by the addition of diazomethane to the steroidal olefin followed by decomposition of the intermediate 16 alpha , 17 alpha-pyrazoline. The D'5-pentarane was obtained by conventional contraction of cyclohexane D'-ring of the corresponding D'6-pentarane. Progestational and contraceptive activity has been investigated for these compounds. They were found to exhibit a high progestational activity in the McPhail assay and also to be active in the pregnancy maintenance test in ovariectomized rabbits. Some of the D'-pentaranes displayed a remarkable contraceptive effect in combination with mestranol.