The kinetics of the propagation reaction in the polymerization of isoprene initiated by n-, sec-, or t-butyllithium, in the absence or in the presence of the three corresponding lithium butoxides, are studied in the two possible major cases. In the first case, the propagation takes place without simultaneous initiation; the reaction is then first-order versus the monomer concentration and one-sixth-order versus the polyisoprenyllithium concentration. The lithium butoxides decrease the propagation rate, but the effect is not very dependent on the nature of the butoxide. In the second case, the presence of the initiator decreases the propagation rate in a manner which depends on the concentrations of the reactants and on the nature of the initiator. The effect of the lithium butoxide is not simply additive. Depending on the relative concentrations, a synergistic effect may be observed. It is suggested that the active species are chiefly ion pairs of unassociated polyisoprenyllithium. Ion pairs from associated molecules may have limited activity.