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 \approx 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 \pm 1.9 ) \times 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 \pm 3 ) \times 10 ^ { -16 } yr ^ { -1 } .