Poly(ADP-ribosyl)ation is a posttranslational protein modification carried out by a family of enzymes referred to as poly(ADP-ribose) polymerases (PARPs). It has been proposed that the broad nuclear distribution of PARPs may allow them to modulate gene expression in addition to their more accepted role as DNA repair mediators. The role of poly(ADP-ribosyl) ation during oogenesis and folliculogenesis is unknown. Here we found that when 3- to 4-wk-old mice were injected with 5-amninoisoquinolinone, a water soluble inhibitor of poly(ADP-ribosyl) ation, it leads to considerably increased oocyte numbers and a dramatic increase in primordial follicle numbers. Furthermore, we show that inhibition of poly(ADP-ribosyl) ation leads to an increased expression of specific genes and pathways in mouse ovaries, in particular, transforming growth factor superfamily members. Our results demonstrate that poly(ADP-ribosyl) ation, is important in oogenesis and folliculogenesis, and it may have a differential role in regulating gene expression, DNA repair, and apoptosis. The novel function of poly(ADP-ribosyl) ation in oogenesis and folliculogenesis sheds light on the alternative role that DNA repair mediators may play in cellular development and differentiation.