If one assumes that the outgoing lepton produced in a neutral-current event $\ensuremath{\nu}+N\ensuremath{\rightarrow}{\ensuremath{\nu}}^{\ensuremath{'}}+X$ is massive, it might be possible to identify it by its decay products. If the experimental lower limit on the lifetime of ${\ensuremath{\nu}}^{\ensuremath{'}}$ is about ${10}^{\ensuremath{-}9}$ sec, upper limits for its mass are given which depend on the form of the neutral current.