  It is shown that the mass dependence of the $\Lambda$-lifetime in heavy
hypernuclei is sensitive to the ratio of neutron-induced to proton-induced
non-mesonic decay rates R_n/R_p. A comparison of the experimental mass
dependence of the lifetimes with the calculated ones for different values of
R_n/R_p leads to the conclusion that this ratio is larger than 2 on the
confidence level of 0.75. This suggests that the phenomenological $\Delta$I=1/2
rule might be violated for the nonmesonic decay of the $\Lambda$-hyperon.
