Ruelle D. Positivity of entropy production in nonequilibrium statistical mechanics. (50K, TeX) ABSTRACT. We analyze different mechanisms of entropy production in statistical mechanics, and propose formulae for the entropy production rate $e(\mu)$ in a state $\mu$. When $\mu$ is a steady state describing the long term behavior of a system we show that $e(\mu)\ge0$, and sometimes we can prove $e(\mu)>0$.