The effect of cyclophosphamide (CP) injection (60 mg/kg i.p., single dose) on volume growth and tissue oxygenation (pO2 distribution) was investigated in rat DS-sarcomas. CP was administered 4 days after subcutaneous (s.c.) tumor implantation (volume approximately 0.35 ml). Polarographic pO2 measurements were performed in the subcutis at the hind foot dorsum and in tumors 72 h after CP administration. The oxygenation status of these tissues was compared with that of saline-treated controls. CP-injection caused a mean growth delay of 11 days in DS-sarcomas and had no impact on the oxygenation status of the subcutis. In contrast, in s.c. growing DS-sarcomas the pO2 distribution improved significantly when treated tumors (0.59 ml volume) were compared with their untreated counterparts (1.15 ml volume). Comparison of the oxygenation data of CP-treated tumors with size-matched controls revealed an identical oxygenation status in the experimental tumors used. Thus, when 'reoxygenation' is discussed, one should consider whether it is solely the result of tumor shrinkage or a volume-independent phenomenon.