Platelet aggregation was measured in rat plasma at pH values between 6.0 and 8.0 at temperatures, of 21 degrees, 37 degrees and 43 degrees C, respectively, and under aerobic and anaerobic conditions. Platelet aggregation inhibition was predominant at neutral and weakly acidic pH values. At 37 degrees C, aggregation and desaggregation were markedly accelerated as compared to 21 degrees C. In contrast, platelet aggregation at 43 degrees C was not or at most insignificantly increased in comparison to 37 degrees C-Oxygen depletion had no measurable effect. It is concluded from the results obtained that the microcirculation inhibition detected in rats and clinically utilized in the Cancer Multistep Therapy (tumor acidification plus hyperthermia) is highly likely not mediated by platelet aggregation.