The isomerization rates of a calixarene in benzene and in chloroform have been calculated by using molecular dynamics simulations. The reaction coordinate that is employed is based on the unstable normal mode at the saddle point of the potential energy surface. The free energy as a function of this coordinate has been calculated by means of umbrella sampling. Comparison of the free energies in the solvents with those in vacuum reveals that both solvents destabilize the paco conformation and stabilize the transition state region. In chloroform the calixarene shows a stronger preference for the cone conformation than in benzene or in vacuum. The isomerization rate has been determined by the reactive flux method. In benzene the transmission coefficient is about a third higher than in chloroform. The calculated rates are in perfect agreement with experimental data.