  We calculate the sensitivity of the inversion spectrum of ammonia to possible
time-variation of the ratio of the proton mass to the electron mass,
mu=m_p/m_e. For the inversion transition (lambda= 1.25 cm^{-1}) the relative
frequency shift is significantly enhanced: delta(omega)/omega=-4.46,
delta(mu)/mu. This enhancement allows one to increase sensitivity to the
time-variation of mu using NH_3 spectra for high redshift objects. We use
published data on microwave spectra of the object B0218+357 to place the limit
delta(mu)/mu =(0.6 +/- 1.9) 10^{-6} at redshift z=0.6847; this limit is several
times better than the limits obtained by different methods and may be
significantly improved. Assuming linear time dependence we obtain
dot{mu}/mu=(-1 +/- 3) 10^{-16} yr^{-1}.
