A new tumour model that is particularly suitable for testing intraperitoneal chemotherapy is described. Single tumours were induced to grow in the mesentery of rats by the implantation of small pieces taken from subcutaneous tumours. Tumour growth was monitored by repeated laparotomies at which the tumour size was measured with calipers. In this way, growth curves of treated and untreated tumours could be defined. The diameter of untreated intraperitoneal tumours increased linearly with time [diameter (mm) = 0.39 t (days) +2.4]. Tests using different numbers of laparotomies showed that the procedure itself had little influence on growth. Cell kinetic studies of 6-mm tumours showed a mean labelling index of 31% and a volume-doubling time of 3.9 days, resulting in cell-loss factors probably in excess of 70%. The model was tested by assessing the effect of the chemotherapeutic agent cisplatin. Regression and regrowth could be satisfactorily followed, leading to estimates of growth delay. This model therefore provides a quantitative way to assess the response of intraperitoneal tumours to chemotherapy.