This paper studied mechanism of Ce3+, Cd2+, Pb2+ on RNase activity from bovine pancreas. The results showed that the activity of RNase was enhanced under the treatment by Ce3+, Cd2+, Pb2+ at lower concentration (10-60 or 10-30 mumol.L-1), but was inhibited by Ce3+, Cd2+, Pb2+ at higher concentration (40 or 70 mumol.L-1 above), and the inhibition was in the order as Pb2+ > Cd2+ > Ce3+. The equilibrium dialysis demonstrates that RNase may have one Ca(2+)-binding site. The fluorescence titration showed that one molecule of RNase has one binding site for Ce3+, the association constant k for its low-affinity Ce(3+)-binding site is 1.22 x 10(8) L.mol-1. However, it can bind three Cd2+ or Pb2+ and the association causing constant k for its low-affinity Cd2+ or Pb(2+)-binding site is 1.8 x 10(8) L.mol-1, 2.01 x 10(8) L.mol-1, respectively, and caused the conformational changes of RNase.