We have constructed MOSFET-like structures using the quasi-one-dimensional conductors NbS e 3 and Ta S 3. At temperatures below the Peierls transition, an applied gate voltage modulates the threshold field for sliding motion of charge-density waves by up to 40%. The resulting modulation of the collective conductance can be more than 2 orders of magnitude larger than that of the single-particle conductance. We consider several possible mechanisms for this conductance modulation, and suggest electric-field-induced variations of the charge-density-wave order parameter as the most plausible mechanism.