We have measured the ability of the intercalating drugs proflavine, ethidium bromide, actinomycin D, and bismethidiumspermine to inhibit the salt induced transition of poly(dG-dC) from the B to the Z form. While all of the drugs studied slowed the B to Z transition, the effectiveness of the drugs correlates much better with their DNA binding kinetics than their DNA binding constants. In studies where the binding densities of ethidium and actinomycin were varied we have found that high levels of ethidium, more than 1 per 20 base pairs, were required to inhibit the B to Z transition while low levels of actinomycin, less than 1 per 450 base pairs, reduced the transition rate. Studies of the B to Z transition in the presence of both actinomycin and ethidium suggest that the drugs inhibit the transition by different mechanisms. The results are interpreted in terms of a modification of the kinetic model proposed by Pohl and Jovin in which, depending on the DNA binding kinetics of the drug, the drug may inhibit nucleation and/or propagation of the B to Z transition.