A fast computation method for the simulation of an EPR spectrum of dynamically exchanging biradicals and a FORTRAN program are presented here. The lineshape has been developed in the Liouville representation of quantum mechanics following Heinzer′s derivation described for broadened isotropic EPR spectra. Application of the Liouville density treatment to calculate the biradical EPR transitions permits simulation of the intramolecular exchange among many sites. The program DYNBIR can simulate the EPR spectrum in the complete range of slow- to fast-exchange limit involving a maximum of four conformations for the biradical. The program was tested by reproducing the results reported earlier by Parmon and Zhidamirov for nitroxyl biradical. Simulations of the spectrum under weak, intermediate, and strong spin-spin exchange in slow and fast dynamic situations are presented. Application to simulate the temperature-dependent spectrum of a long-chain biradical is also presented. © 1993 by Academic Press, Inc.