The validity of phenomenological equations describing dissipative processes in chaotic regime was tested in this paper. We carried out a direct stochatic simulation of the chemical Lorenz model and found that in this chemical chaos model the deterministic attractor and Poincare maps constructed from it differ drastically from their mesoscopic counterparts of stochastic simulation. These results indicated that, in contrast with previous studies of Williamowski-Rossler model, intrinsic fluctuations do have pronounced effect in the chaotic regime in the case of chemical Lorenz. The fact that the effect of intrinsic fluctuations on macroscopic dynamics differ from case to case proposes that the issue whether phenomenological equations are valid or not in the chaotic regime deserves further studies in the future. (C) 1997 American Institute of Physics. [S0021-9606(97)50341-2].