Positivity of entropy production

We discuss the positivity of the mean entropy production for stochastic sys tems driven from equilibrium, as it was defined in refs. 7 and 8. Non-zero entropy production is closely linked with violation of the detailed balance condition. This connection is rigorously obtained for spinflip dynamics. W e remark that the positivity of entropy production depends on the choice of time-reversal transformation, hence on the choice of the dynamical variabl es in the system of interest.