Fusion-fission systems in which the tritium production rate in the fusion reactor is variable and in which tritium can be bred in the fission reactors are evaluated. The calculational model determines the feasible power production ratios among the system components and an economic figure of merit for feasible (fuel self-sufficient) systems. Allowance for system growth is accomplished with the use of a doubling time parameter. Results are computed for systems with LWR power reactors, modest and advanced tritium production reactors, and both high and low fuel production capabilities for the fusion reactor.