The theory of chemically induced dynamic nuclear polarization (CIDNP) of radical pairs in restricted volume formed in experiments with instantaneously switched external magnetic field is developed. For this purpose the theory of recombination of radical pairs with instantaneously changed spin Hamiltonian is extended to the case of rather weak magnetic fields. Using kinematic approximation it is shown that the recombination probability of radical pairs is fully expressed via the matrix formulas in terms elf averages of Green's functions of Liouville equation over both the reaction zone and that of energy level crossing. The effect of field switching on the CIDNP value is calculated for the typical parameters of the radical pairs. Simple biexponential expressions are derived or the CIDNP time dependencies for the case of strongly averaged S-T-0 transitions and full mixing in configuration space. (C) 1997 American Institute of Physics. [S0021-9606(97)51345-6].