The Rydberg state decay dynamics of ammonia is investigated in real-time by (2 + 1') femtosecond pump-probe ionization detection. The lifetime of ammonia in the (E) over tilde (1)A'(1) state is determined as 936 +/- 92 fs for the first time. The internal conversion (IC) pre-dissociation mechanism of the (E) over tilde'(1)A'(1), state has been discussed in this report. Concurrently, the (A) over tilde (1)A"(2) state (v'(2) = 4) lifetime is estimated as 51 +/- 4 fs, which is identical with the previous reports. (C) 2002 Elsevier Science B.V. All rights reserved.