Interstitial fluid pressure (IFP) has been reported to be inversely correlated to rate of perfusion and oxygen tension (pO2) in experimental tumours (Lee et al., 1992; Roh et al., 1991 a). Studies of patients with squamous cell carcinoma of the uterine cervix have provided clinical data consistent with the experimental data (Roh et al., 1991b; Milosevic et al., 1995). These observations have led to the hypothesis that IFP might be a useful indicator of tumour oxygenation status. The purpose of the work reported here was to examine in detail the general validity of this hypothesis. R-18 human melanoma xenografts grown intradermally in Balb/c nu/nu mice were used as tumour model system. IFP and perfusion rate or IFP and pO2 were measured in the same individual tumours in two independent series of experiments. The wick-in-needle method was used to record IFP. Perfusion rate was studied by using the 86Rb uptake method. The KIMOC-6650 Eppendorf histograph was used to measure pO2. IFP, perfusion rate and pO2 differed considerably between individual tumours. However, there was no relationship between IFP and perfusion rate or IFP and pO2, suggesting that the oxygenation status of tumours cannot be derived from measurements of IFP. Consequently, IFP is probably not a useful predictor of radiation resistance caused by hypoxia.