In the intracerebral 9L rat gliosarcoma, the spatial distribution of the photosensitizer haematoporphyrin derivative (HpD) was studied after intratumoral injection. The fluorescence volume was measured in histological sections from 10 min up to 5 days after injection. Complete sensitization of the tumours could not be achieved by slow stereotactical injection of 4 mm3 HpD (mean HpD fluorescence volume, 13 +/- 11 mm3). Larger parts of the tumour could be loaded with HpD (39 +/- 23 mm3, p = 0.0001) by increasing the injection velocity and the volume to 50 mm3. Again, complete sensitization of the tumours was not achieved during a time scale of 5 days after intratumoral injection. Although the fluorescence volume did not change significantly with time, it was influenced by the injection site within the tumour. Injection of HpD within 1 mm from the tumour border resulted in significantly smaller fluorescence volumes in the tumour than injection into the tumour centre. Large injection volumes caused an increased leakage of HpD to normal brain, leading to the loss of selectivity of photosensitizer content and the occurrence of dark toxicity of normal brain while the tumours still appeared vital.