Using a multichannel laser Doppler system equipped with custom-developed microprobes, we have evaluated real-time fluctuations in microregional blood flow in two experimental murine tumour systems. The results show that in both the sarcoma F and the carcinoma NT over 50% of the microregions evaluated show a fluctuation in red blood cell flux by at least a factor of 2 over a 60 min time period. Approximately 20% of the regions monitored demonstrated a change in flow by a factor of 5 or more. Within the 1 h monitoring period, approximately 20% of the changes were reversed (SaF, 21%; CaNT, 19%). The duration of change for these regions ranged from 6 to 45 min. Similar temporal fluctuations in flow were seen in anaesthetised and unanaesthetised animals, indicating that artifacts due to probe movement were minimal. These findings clearly demonstrate that fluctuations in microregional erythrocyte flux are a common feature of the experimental tumours studied.