We determined the activity of phosphofructokinase (PFK), a rate-limiting enzyme of glycolysis, in the rat oocyte during maturation either by luteinizing hormone stimulation in vivo or by cultivation under various conditions. In vivo, the activity of PFK in the maturing oocyte increased significantly, suggesting that the glycolytic pathway developed as the first meiotic division proceeded. A breakdown of the germinal vesicle and an increased activity of PFK were observed when denuded oocytes were cultured. Treatment of the oocytes with dibutyryl cAMP and 3-isobutyl-1-methyl xanthine almost completely inhibited germinal breakdown, whereas the activity of PFK increased significantly. The data suggest that regulation of PFK activity is not directly dependent on cAMP or cAMP-dependent protein kinase, and is not necessarily dependent on meiotic maturation. Our study also revealed that epidermal growth factor (EGF) significantly increased PFK activity, showing that EGF may be a potent inducer of oocyte maturation.