Experiments concerning the dependence of respiration of Arenicola marina (Annelides Polychaeta) on size and temperature. A knowledge of the rate of oxygen consumption is very important for the evaluation of many physiological and ecological problems. Among the many factors affecting respiratory rate, water temperature and body size are particularly considered here. The modifying effects of body size may be expressed mathematically by the allometric formula: y = b • w% where b represents the rate of O~ consumption of an individual whose weight is expressed in a &osen metrical weight unity (i. e. in grams, ounces, etc.), and a represents the decrease of metabolic rate during growth. In Arenicola the exponent is not the same at all temperatures tested. In most cases it lies between 0.7 and 0.8, i. e., between a weight proportional respiratory rate and a surface proportional one. Minimum values for a were found in experiments conducted in summer at 200 C and in spring at 15 ° C. They &aracterize an optimum efficiency of metabolism at these temperatures. Determinations of b demonstrated that metabolic rate of ArenicoIa is significantly less affected in spring (10 ° to 20 ° C) and autumn (10 ° to 25 ° C) than is usually known from biological processes. However, the temperature coefficients above and below these temperature ranges are very high. Another break in the temperature-rate curve could be demonstrated below 5 ° C in spring.