Cuttlefish (Sepia officinalis) display diurnal changes in activity levels that are linked to the gas loading of the cuttlebone, which regulates buoyancy (Denton and Gilpin-Brown, 1961).To investigate diurnal changes of standard and active metabolic rates, we analysed activity patterns of S. officinalis using new techniques of video tracking. We monitored S. officinalis activity by video continuously for 5 days. In a parallel experiment we measured their rates of oxygen consumption in order to explore whether metabolic rate changes in parallel with activity levels. Both measurements of activity and oxygen consumption indicate a strong diurnal pattern with a maximum physical activity shortly after midnight and a relatively constant minimum value during daytime.In order to determine whether patterns of metabolic regulation go hand in hand with the diurnal rhythm of activity-dependent metabolic slowing (daytime) and stimulation (nighttime), we analysed the activities of enzymes involved in standard and digestive metabolism in mantle and hepatopancreas tissues to complete the picture. Moreover, we could show that different feeding schemes strongly affect the distinctiveness of the diurnal activity pattern.Supported by NERC grant NERC/A/S/2002/00812.