We compute the transition probability per unit time and volume relative to processes of the general type a1 + a2 → a3 + a4 + v + v, where a1, a3 are charged baryons or leptons and a2, a4 are leptons, in view of estimating the neutrino energy loss rate in stars due to these processes. A general expression is given for the neutrino energy loss rate in the case of a nondegenerate gas of fermions. © 1969, Società Italiana di Fisica. All rights reserved.