The gross anatomy and histology of cultured gilthead sea bream symptomatic for the winter syndrome are severely compromised. To determine how these modifications affect the absorptive capacities of the intestine, we exposed one group of gilthead sea bream to 8ordm;C and subjected a second group to fasting at 18ordm;C (forced fasting) (Phase 1). After one month, initial conditions were restored and fish were allowed to recover for 20 days (Phase 2). A third group was maintained at 18ordm;C and fed regularly throughout the experiment (control). Samples of proximal and distal intestine were taken at the end of each phase and studied independently. Na⁺/K⁺ ATPase activity of intestine wall homogenates was determined spectrophotometrically, and uptake of [14C]D-glucose and [3H]phlorizin binding were measured in brush border membrane vesicles (BBMV). During Phase 1, Na⁺/K⁺ ATPase activity along the intestine of forced fasting fish decreased up to 38%, whereas exposure to 8ordm;C resulted in a 50% increase. Phlorizin binding and D-glucose uptake in BBMV from proximal and distal intestine of 8ordm;C fish increased, while those of forced fasting animals dropped. By the end of Phase 2, both groups showed control values of Na⁺/K⁺ ATPase activity and phlorizin binding. Glucose uptake rates of 8ordm;C fish returned to control values, but those of forced fasting fish remained elevated, indicating reserves might not yet be fully replenished. Coinciding with a raise in natural photoperiod, Na⁺/K⁺ ATPase activity and D-glucose transport of control fish decreased throughout the experiment, suggesting the occurrence of seasonal variations in the intestinal function of gilthead sea bream.