Published results on the relative contribution of food and water in the accumulation of 65Zn by freshwater fish are contradictory and it is observed that the retention rate of radiozinc absorbed from food varies over a wide range (from 5 to 70%) depending on the authors (Chipman et al., 1958; Nakatani and Liu, 1964; Pentreath, 1973, 1975; Berg and Brazzelli, 1975; Merlini et al., 1976; Baudin, 1981). The present paper aims to highlight the variability of the retention rate of the radionuclide absorbed from contaminated food. Two experiments were carried out using the soft tissue of previously contaminated lymnaea as food. The first experiment was conducted over a 9 week period on three groups of carp, each receiving food of identical composition, the only difference concerning the splitting up of the weekly food intake. The second experiment was carried out over a 40 day period on a group of 15 carps which received 26 daily meals (Table I). In the first experiment, the pattern of contamination is similar in the three cases, but the carps in groups A and B were more contaminated than those in group C (Table 2, Fig. 1). The same observations apply to the trophic transfer factor (Fig. 2). These differences are explained by a lower retention of 65Zn absorbed with each of the 9 weekly food intakes for the carps in group C than for those in groups A and B (Table 3). The global rate of retention of radiozinc develops in accordance with an exponential model (Fig. 3), reaching maximum value of approx. 14% (group C) to 25% (groups A and B). The similarity of the results for groups A and B can be explained by the fact that the difference in the distribution frequency of the food constituting the weekly intake (2 and 4 days) is insufficient to bring about a significant difference in either the transit rate or the digestibility of the food (Grayton and Bremish, 1977; Luquet et al., 1981; Hudon and De la Noue, 1984). In the case of group C, the lower value of the global retention rate can be explained by an incomplete absorption of the nutriments due to a faster food transit rate in the digestive tract (Possompes et al., 1975; Hudon and De la Noue, 1984). In the second experiment, correlatively with the strong contamination of the ingested food, an increased accumulation of 65Zn is observed in the carp. However, contrary to what was observed in the first experiment, the global rate of retention decreases as a function of time (Table 5). The differences which can be observed between the results of the two experiments do not only concern the level but also the development of the parameters. Thus, for the concentration (Fig. 4a) and the trophic transfer factor (Fig. 4b) the evolution corresponds to power functions, but the slope of the curves indicates that the kinetics are different. This is the consequence of the conflicting evolution of the retention rate 65Zn by the carp in the two experiments (Fig. 4c). The homeostatic regulation of zinc, proposed by Berg and Brazzelli (1975), cannot justify the observed difference in retention of 65Zn. The hypothesis concerning the influence of the nature of the food proposed by several authors (Pentreath, 1975; Berg and Brazzelli, 1975; Merlini et al., 1976) can hardly explain such differences. According to an analysis of results obtained during several experiments (Table 6), it can be also hypothesized that the rate of retention of radiozinc by the carp is inversely proportional to the quantity of radionuclide absorbed with the food, due to a slow turnover of zinc. The faeces are a very important pathway of the radionuclide excretion (Table 4). This fact agrees with high radioactivity levels measured in the digestive tract of fish contaminated from water or placed in decontamination for several days or weeks (Hibiya and Oguri, 1961; Joyner, 1962; Pentreath, 1975; Aprosi, 1978; Suzuki et al., 1979; Baudin, 1977, 1981).