For the Shulz distributions with weight-polydispersity from 1.4 to 2.0, the following equation is correct : ln[eta] = ln[eta](0) - a{ln P-0 + DD - ln[P-F - (P-F - P-0) X exp(-1.2 X DD)]}. Here, P-0 and P-F are viscosity-polydispersities of the initial molecular weight distribution (MWD) and the Flory distribution, respectively; a is the Mark-Houwink exponent; and DD = ln(M(n0)/M(n)). For more narrow MWDs, the complicated equation is required : ln[eta] = ln[eta](0) - a{ln P-0 + DD - ln[P-F - (P-F - P-0) X exp(-0.9 X DD - 0.28 X DD2)]}. The inaccuracy of these equations is less than 1.5%.