The perfusion characteristics of the P22 carcinosarcoma were investigated in tissue-isolated tumour preparations in the ovarian and inguinal fat pads of BD9 rats. Tumours were perfused with a physiological buffer of known viscosity and changes in perfusion pressure were recorded at different perfusion rates in an ex vivo system. At perfusion pressures exceeding 30-40 mmHg tumour flow rate was directly proportional to the perfusion pressure in all tumours, indicating a constant resistance to flow. An apparent positive pressure difference across the tumour vasculature of 20-30 mmHg occurred under conditions of zero flow in either site. At low perfusion pressures, the flow resistance increased sharply due to increases in the geometric resistance of the tumours. These findings are in accord with previously published data. Geometric resistance increased with tumour volume in both sites and was approximately five times greater in the inguinal tumours than it was in the ovarian tumours, on a weight to weight basis. The dependence of tumour geometric resistance on perfusion pressure differs from the situation in normal tissues and may provide a means of manipulating the tumour microcirculation to the exclusion of the systemic blood supply. The dependence of geometric resistance on tumour site may partly explain why tumours located in different sites respond differently to various forms of therapy.