  Heavy neutrinos were sought in pion decays $\pi^+ \rightarrow \mu^+ \nu$ by
examining the observed muon energy spectrum for extra peaks in addition to the
expected peak for a massless neutrino. No evidence for heavy neutrinos was
observed. Upper limits were set on the neutrino mixing matrix $|U_{\mu i}|^2$
in the neutrino mass region of 15.7--33.8 MeV/c$^2$, improving on previous
results by an order of magnitude.
