An experimental model has been devised to measure within the body of a single animal various parameters required for rational cancer chemotherapy. The growth and regression of tumor cells has been tested using cyclophosphamide as an effective chemotherapeutic agent. It was found that the tumor grew at an average rate of 2%/hr and could be regressed about 2%/hr. The rates of growth between s.c. and i.v. implants were slightly different as were their rates of regression under cyclophosphamide therapy (whether this depends on initial size of the tumor at the time cyclophosphamide was given has not been investigated). The model can be used to predict the starting populations of a tumor inoculum (t0) as well as the time it will take to destroy the last single viable tumor cell in an individual mouse. © 1974, American Association for Cancer Research. All rights reserved.