Résumé : The first measurement of the neutral current inelastic cross-section ratio σ(ν + n → v + ...)/σ(ν + p →v + ...) is presented. It is based on a sample of 573 neutral current events from the Gargamelle propane experiment using the CERN-PS neutrino beam. The ratio is found to be 0.76 -0.15 +0.17. The relative contributions of reactions on neutrons and protons are estimated from the charge and proton multiplicity distributions. This method was already successfully applied to charged current channels where an independent measurement of the cross-section ratio is possible. © 1978.