Since the 1980s, additional studies have examined phosphodiesterase inhibitors and their role in prevention of oocyte maturation. Tsafriri and colleagues showed that phosphodiesterase inhibitors were able to exert their effects selectively via type specific isoforms. This study revealed that granulosa cells exert effects via type 4 phosphodiesterase inhibitors and oocytes respond to signals via type 3 phosphodiesterase inhibitors such that oocyte maturation was prevented in the presence of PDE3 inhibitors yet ovulation was unaffected (15). Most recently, Jensen and colleagues examined both in vitro and in vivo effects of a phosphodiesterase 3 inhibitor ORG 9935 on oocyte maturation in rhesus monkeys. In these animal studies, it was found that in vitro, ORG 9935 at a concentration of 1.0umol/l was able to completely inhibit oocyte maturation (7).