A radiotracer technique involving 51Cr was used to monitor the egestion rates of A. fluviatilis and P. contortus. When snails were starved, either before or after exposure to labelled food, gut emptying rate reduced. Examination of faecal components suggested that this was due to reductions in the rate of passage of food through the hepatopancreas. Since the latter is the major site of digestion and absorption in the Gastropoda this strategy was explained as an attempt by the snails to extract more nutrient from the disturbed food supply. Other factors affecting egestion rate were, temperature, food quality and snail size. Weight for weight egestion was more rapid in P. contortus than in A. fluviatilis and this difference is typical between detrivores and herbivores. Information on defaecation strategy enabled an estimation of field absorption and ingestion rates in A. fluviatilis. There was good agreement between the latter and estimates predicted on the basis of laboratory observation. However, field determined absorption efficiencies were more constant and generally higher than those predicted on the basis of laboratory measurements.