Interstitial fluid pressure (IFP) in DMBA-induced rat mammary tumours was measured by micropuncture with sharpened glass capillaries (diameter 1-3 micron) and by the 'wick-in-needle' technique (WIN). IFP in the superficial layers of the tumours (depth less than 800 micron) was measured with micropuncture, while WIN was used for measurements in deeper areas. IFP in the superficial layers of the tumours was 2.4 mmHg (SD 2.4, n=19), while IFP in deeper layers increased with increasing tumour weight. Thus central IFP in tumours weighing more than 5.5 g was 16.0 mmHg (SD 4.8, n=10), with a maximum value of 23.3 mmHg. When related to tumour histopathology a significant correlation was found between high cellular differentiation (i.e. low degree of malignancy) and high interstitial fluid pressure only. The high IFP may be secondary to a high capillary protein premeability and/or a relatively insufficient lymphatic drainage. A further rise in IFP might result from ischaemic cell swelling and/or a continuous cell proliferation within a tissue of rather low compliance. Together or alone one or more of these factors could facilitate the development of a compartment syndrome with concomitant cell death and tissue necrosis.