Summary. The first cleavage of embryos derived from random-bred, inbred, and hybrid-inbred female mice was not arrested by purines at concentrations as high as 30 μm. Development after the first or second cleavage was arrested by hypoxanthine, adenosine or inosine, but not guanosine. In agreement with previous results, the purine-induced block was reversed when arrested embryos were transferred to purine-free media after 24 h in culture. The cleavage arrest was not due to elevations of cAMP as a result of inhibition of phosphodiesterase activity since similar concentrations of phosphodiesterase inhibitors or dibutyryl cAMP did not block development. Treatment with inhibitors of enzymes that convert IMP to AMP or to GMP did not reverse the hypoxanthine-induced block, thus demonstrating that mitotic arrest is mediated by a mechanism different from the hypoxanthine arrest of meiosis. Thymidine incorporation studies showed that the block did not prevent the onset of DNA synthesis. The results reveal a profound sensitivity to purine inhibition of a cell process that occurs during the first 30 h of mouse embryo development and is necessary for progession through the G2 or M phases of the second or third cleavage.