Dielectric normal mode relaxation was studied on blends containing a small amount (similar to 5 wt %) of probe cis-polyisoprenes (PI) in a series of polybutadienes (PB) of different molecular weights M(B). Since the PI chain has Stockmayer’s type-A dipoles parallel to the chain contour, while PB chains do not, we can study global motion dynamics of the probe chain under various entanglement conditions between PI and PB chains. The dielectric normal-mode relaxation time tau of PI with molecular weight M(I) followed a power law relation tau proportional to M(I)(a) with alpha varying from 2 to 3 depending on Mg : For M(B) < 3000, alpha = 2.0 +/- 0.1 in agreement with the Rouse model, while for M(B) > 20 000, alpha = 3.0, conforming to the pure reptation model, and in the range 3000 < Mg < 12 000 a increased rather sharply with M(B). When tau was reduced to an isofriction state tau zeta, the log tau zeta versus log M(I) plots for different M(B) converged at M(I) = 1900 (congruent to the entanglement spacing M(eB) of bulk PB). The slope a of each log tau zeta versus log M(I) plot at constant M(I)/M(B) (>1) was larger than 3 and became 3.7 (observed for monodisperse PI melts) at M(I)/M(B) congruent to 2-3. These findings were consistently interpreted with Graessley’s constraint release model. The shape of the dielectric loss curves (reflecting the distribution of relaxation times) of the blends agreed rather well with the Rouse model when M(I)/M(B) > 10 and Mg < M(eB), but the loss curves became broader with decreasing M(I)/M(B) (or increasing M(B) > M(eB)) especially in the high-frequency tail, as usually observed in monodisperse P-I melts.