We report dielectric normal-mode relaxation of probe cis-polyisoprenes (PI) in dilute blends of PI and polybutadiene (PB) with broad molecular weight distributions (MWD). The PB matrixes included two binary PB blends of narrow-MWD PBs and one mixture of seven narrow-MWD PB fractions. Results were compared with our previous results on narrow-MWD PI/PB blends (Macromolecules 1995, 28, 3588) in which the double-logarithmic plot of the longest relaxation time tau of PIs against their molecular weights, M(I), conformed to a straight line with the slope varying from 2 (the Rouse behavior) to 3 (pure reptation) with increasing MW, M(B), of the matrix PBs. The plots for the broad-MWD blends also conformed to straight lines, indicating that the broad-MWD matrix is equivalent to a narrow-MWD matrix of an equivalent molecular weight M(equiv). However, as compared with the same weight-averaged M(B), the slopes are slightly smaller than those of the narrow-MWD blends, suggesting that the small MW matrix chains accelerate the rate of relaxation of the probe chain. The behavior cannot be explained quantitatively by the constraint-release model proposed by Graessley. We also examined the shape of the dielectric loss epsilon " versus frequency f curve. All epsilon " curves observed here are broader than the prediction of the Rouse and Doi-Edwards theories which predict that the slope of the log epsilon " vs log f curve is -0.5 on the high-frequency side of the loss peak.