In rats several circadian rhythms such as heart rate, body temperature, and locomotor activity are known. Several authors found a loss of day-night-rhythm (locomotor activity, EEG) after portacaval shunting (PCA). The aim of this study was to evaluate whether muscle PO2 oscillations are circadian and whether they are altered after time-limited hypercirculation caused by PCA. 126 days after operation tissue PO2 of m. rectus abdominis of 9 rats with PCA and 10 controls was measured with a multi-wire surface electrode. All animals were kept under constant conditions and each animal was measured 6 times at intervals of 4 hours in order to get a circadian PO2 course. In controls the circadian course of mean muscle PO2 resembled a sine oscillation with high values at night and low values in the afternoon. In PCA-rats the time course of mean muscle PO2 showed 3 oscillations with different amplitudes, each with a period length of 24 hours. Our results indicate that oscillations of muscle PO2 are determined principal by circadian locomotor activity and that time-limited hypercirculation influences the circadian course of mean muscle PO2.