  A method and results of an experimental test of the time stability of the
half-life of alpha-decay Po-214 nuclei are presented. Two underground
installations aimed at monitoring the time stability have been constructed.
Time of measurement exceeds 1038 days for one set up and 562 days for the
other. It was found that amplitude of possible annual variation of Po-214
half-life does not exceed 0.2% of the mean value. The limit on the deviation of
the decay curve from exponent at 0.034\times T_{1/2}<t< 0.1\times T_{1/2} range
has been found.
