Discussed is the role of the environmental fluctuations and/or extrinsic oscillatory fields in the chemical equilibria. It is well known that the simple relation between thermodynamic equilibrium constant, kinetic rate constants, and equilibrium concentrations of reagents holds for ideal systems only, and breaks down for nonideal ones. We show that when fluctuations and/or oscillating fields play an essential role in the chemical kinetics, this simple relation breaks down even for ideal systems. Uni- and bimolecular reactions with mass-action kinetics (ideal systems) are considered in detail, for time-dependent periodic (sinusoidal or square-wave) perturbations and random dichotomous ones. It is shown that such perturbations (of zero mean) of the kinetic reaction rate parameters k(j), although they leave unchanged the thermodynamic equilibrium constant K-eq, at the same time may change considerably the "kinetic" one K-kin defined as the ratio of mean (averaged over oscillating perturbations) equilibrium concentrations (raised to powers equal to their stoichiometric coefficients) of products and substrates. Equilibrium concentrations in a closed reactor, and the reaction yield (output concentrations) in a flow reactor are closely related to each other. Stationary-in-the-mean perturbed yield is being calculated and shown to be changed by time-dependent changes of either reaction rate coefficients or input and output of reagents. The values of kinetic parameters may be altered in some situations by changes of physical parameters such as light intensity or electrode potential. The input and output of the flow reactor is easily controlled. This creates the possibility of pushing the chemical reactions in a desired direction, which can be of importance both in theory and in applications, and also enables the experimental verification of our results, especially in the flow-reactor conditions. (C) 2001 American Institute of Physics.