Denton and Gilpin-Brown hypothesized in 1961 that the cuttlefish Sepia officinalis displays diurnal changes in activity levels. We therefore analysed activity patterns of S. officinalis using new techniques of video recordings and analysis. 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 see whether metabolic rate changes in parallel with activity levels. Preliminary tests support the original hypothesis of a diurnal pattern. Measurements of oxygen consumption indicate maximum physical activity between 0 and 3 am and a relatively constant minimum value during daytime. Enzyme activities are currently being analysed in mantle and hepatopancreas tissues 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).Supported by NERC grant NERC/A/S/2002/00812.