Although constant-pressure molecular-dynamics simulations can be performed through the use of constraint methods, achieving a desired pressure can be difficult. A technique for setting the pressure is proposed in this paper. The pressure is fixed via an automatic, differential pressure controller which provides a first-derivative coupling to external pressure perturbations. This generates neither pressure overshoot during transient periods nor unrealistic fluctuations at equilibrium. The implementation of this algorithm is outlined and discussed. Results of simulations of a Lennard-Jones fluid are presented.