A biophysical model has been applied to study the kinetics of chromosome exchange formation in human cells. Chromosomal exchange induction (for example dicentrics) by ionising radiation was modelled by means of the Monte Carlo technique. This involved energy deposition by electrons, production of chromosomal breaks (assumed to be DNA double-strand breaks) and their repair and exchange. Exchanges were assumed to result from pairwise interaction between two DNA breaks in a distance-dependent manner. The rate at which exchanges are formed was found to depend upon how the exchange to no-exchange probability ratio varied with time. The assumption that this ratio did not alter with time produced a time constant for the formation of exchanges which was exactly half that of the repair time constant. Longer time constants could not be accommodated unless the probability ratio for exchange increases with time. Different time constants for inter- and intratrack exchanges could be achieved on the basis of DNA double-strand break separation.