Search for massive neutrinos in pi -->e nu decay.

We describe the details of an experiment designed to search for the admixture of massive neutrinos into electron-flavor neutrinos by observation with high resolution of the positron momentum spectrum in {pi}{r arrow}{ital e}+{nu} decay. No indication was found for such an admixture and the corresponding mixing amplitude was constrained below the {ital U}{sub {ital e}{ital i}}{sup 2}{le}10{sup {minus}6} region for neutrinos of {similar to}30 MeV/{ital c}{sup 2} mass.