Taking into account the terms similar to m(2) in the effective chiral Lagra
ngian, we show that, at theta=pi, the theory with two light quarks of equal
mass involves two degenerate vacuum states separated by a barrier. For N-f
= 3, the energy barrier between two vacua appears already in the leading o
rder in mass. This corresponds to the first order phase transition at theta
= pi. The surface energy density of the domain wall separating two differe
nt vacua is calculated. In the immediate vicinity of the point theta = pi,
two minima of the potential still exist, but one of them becomes metastable
. The probability of the false vacuum decay is estimated. [S0556-2821(99)00
711-0].