The present invention relates to an improved process for producing naltrexone [17- (cyclopropylmethyl)-4,5a-epoxy-3,14-dihydroxy-morphinan-6-one] from noroxymorphone [4,5- a-epoxy-3,14-dihydroxy-morphinan-6-one] by alkylation with a cyclopropylmethyl halide.