1. A technique is described for the measurement of the extent of 65Zn absorption by different regions of the intestine in the intact rat. Using this technique it was shown that the duodenum contributed 60%, the ileum 30% and the jejunum 10% to the over-all absorption of 65Zn. Negligible absorption of 65Zn occurred from the caecum and colon. 2. Using ligated loops of rat duodenum in situ, 65Zn absorption was shown to be rapid, with 1% of a 5 microgram dose being transferred to the carcase within 1 min of intraluminal dosing. 3. When 65Zn was injected into ligated loops in a dose range of 1--200 microgram Zn/ml the rate of absorption was linear with respect to time over the first 15 min. The rates of 65Zn absorption v. dose of 65Zn exhibited saturation kinetics indicating absorption by a 'carrier' or enzyme-mediated process. 4. The binding of 65Zn to loop tissue showed biphasic kinetics which suggested that at low intraluminal concentrations of Zn (1--50 microgram Zn/ml) binding was to specific sites whereas, at higher concentrations (50--200 microgram Zn/ml), non-specific binding occurred. 5. A study of the fate of mucosally bound 65Zn showed that over the first 30 min a proportion of the 65Zn was rapidly transferred to the carcase and this was probably associated with the rapid phase of 65Zn absorption described previously. From 30 min up to 6 h after the initial binding, 65Zn was also transferred to the carcase albeit at a much slower rate indicating a slow phase of Zn absorption. A study of the kinetics of this slow phase indicated that the loss of bound 65Zn to the body was a saturable process indicating an enzyme- or 'carrier'-mediated process. A comparison of the kinetics of the slow and rapid phases of 65Zn absorption suggests that these processes are distinct. 6. Histological examination of mucosal tissue of loops exposed to 200 microgram Zn/ml revealed to discernable diamage. Similarly, no effect was observed on either arginine or glucose uptake by isolated duodenal loops in situ, although this concentration of Zn completely abolished fluid uptake. A study of the effect of different doses of Zn showed that 50 microgram Zn/ml inhibited mucosal fluid uptake by more than 50% and 100 microgram Zn/ml by more than 90%. It was concluded that these effects were due to a specific action on the fluid-transfer process and not due to a general poisoning of the mucosa.