This study was undertaken to investigate effects of granulosa cells on mejotic maturation of porcine oocytes in vitro. The results obtained in this study were summarized as follows : The germinal vesicle breakdown(GVBD) rates were 91.5, 93.3 and 96.6%, respectively, when the cumulus oocy:e cornplexes(COC) in the TCM-199 medium with sodium bicarbonate, Na pyruvate, penicillin G, streptomycin sulfate and 10% FCS were cultured in the condition of FSH(0.02 Au/ml), LH(10 $mu$g/ml) and FSH + LH added. And when the COC were co-cultured with granulosa cell (5$ imes$ 106 cells /ml) in the condition of FSH, LH and FSH + LH added, GVBD rates were 94.3, 92.9 and 98.9%, respectively. However, when the COC were cultured in the condition of hormone free and co-cultured with granulosa cells in the condition of hormone free, the GVBD rates were 40.4 and 86.3%, respectively. The GVBD rates were 41.0, 62.7, 84.6, 88.1 and 93.6%, respectively, when the COC were co-cultured with granulosa cells that the concentrations are 0 cells /ml, 1 $ imes$ 106 cells /ml, 5:: 106 cells /ml, 1$ imes$ 107 cells /ml and 5$ imes$ 107 cells /ml.