Rhizopus oryzae IFO 4697 was found to produce intracellular ribonuclease (RNase), and its growth and activity could be regulated under selected metal ion stress. The addition of Fe2+, Mg2+ and Zn2+ to the SLSR medium was essential to growth and RNase production. Ca2+ and Mo6+ stimulated RNase production. It is concluded that the addition of 100 mg/ml Ca2+, 5 mg/ml Mo6+, 0.7 mg/ml Zn2+, 2 mg/ml Fe2+, and 49 mg/ml Mg2+ to the SLSR medium was the best condition for producing RNase in high specific activity (3780 U/mg protein). This result indicates that a metal ion-regulated liquid medium is an efficient culture method for RNase production