1. The standard rate of oxygen consumption, ventilatory frequency and heart rate of adult Lampetra fluviatilis were measured during the light phase of the photoperiod and at times corresponding to various stages in the upstream migration. 2. All three parameters increased during the spawning run but only in mature individuals were significant differences found between the sexes. 3. The regression coefficients for the logarithmic relationship between oxygen consumption and body weight of immature animals were 0.912 and 0.925 at 9.5 and 16 degrees C respectively. 4. Both the standard rate of oxygen consumption and the amount of oxygen taken up during activity increased greatly during the hours of darkness. 5. Oxygen consumption, ventilatory frequency and, to a lesser extent, heart rate increased significantly at 9.5 degrees C over the 100–20% range of saturation with air. 6. Below 20% saturation with air, lampreys no longer remained attached by their oral disc for prolonged periods and the ventilatory frequency rose even more rapidly to reach a maximum of 175 beats/min at 12.5%. Exposure to 7.5% resulted in death within 5–8 h.

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