We present an implementation of the conductor-like screening model (COSMO) within the framework of Car-Parrinello ab initio molecular dynamics. In order to obtain the accurate forces needed for energy-conserving dynamics, analytic derivatives with respect to the atomic positions are required for all energy terms. We use a steep, but continuous surface function that effectively switches the surface charges off when they are not exposed on the molecular surface. This allows us to construct the cavity surface in such a way that the required analytic derivatives of the surface charges and surface segments are always available. Furthermore, we treat the surface charges as fictitious dynamic variables within the extended Lagrangian approach, solving the electrostatic problem determining the charges "on the fly" as the system evolves in time. Our implementation makes it possible to perform energy-conserving ab initio molecular dynamics simulations in which continuum solvation is included. It provides solvation energies within the accuracy expected for a COSMO implementation at the density-functional level and allows one to study special features of reactivity that can only be observed at finite temperature in solution. (C) 2003 American Institute of Physics.