The effects were studied of molecular weight, shear rate, and temperature on the viscous behavior of various linear, monodisperse polyisoprenes of about 90% cis-1,4 structure. Viscosities of from 1010-105 poise were measured on a double cone and plate viscometer while viscosities of 104-10−2 poise were measured on rotating cylinder viscometers. For such polyisoprenes the variation of Newtonian (zero shear) viscosity η with the weight-average molecular weight in the molecular weight range of 2 × 104-1 × 106 may be represented as where A is a temperature-dependent parameter given by The variation of the apparent viscosity ηa with shear rate is also determined and a combined equation may be formulated as This equation is compared with other expressions for the variation of viscosity with molecular weight, temperature, and shear rate.