Slapton Ley is a freshwater hyper-eutrophic lake of two basins connected by a narrow channel. One part of the lake experiences summer blooms of cyanobacteria and poor water quality, including elevated water pH (maximum pH recorded=10.54), the other part is shaded by reed beds, and remains clear and neutral. This study used laboratory and field physiological measurements together with radio-tracking to investigate the potential impacts of alkaline pH on the physiology and behaviour of fish from Slapton Ley. Exposure of perch (Perca fluviatilis) from Slapton Ley to pH 9.50 water in the laboratory caused an immediate inhibition of sodium uptake and ammonia excretion to 34 and 32% of control levels, respectively. Net sodium balance recovered by day 3 of exposure whereas ammonia excretion only partially recovered to 60-70% of the control value from 8 h onwards. Urea excretion did not increase as a result of high pH exposure. Fish from the alkaline part of the lake (pH 9.90) had almost three-fold greater plasma ammonia compared to fish from neutral waters, indicating a pronounced disruption of ammonia excretion in the field. There was no significant disturbance to plasma sodium, chloride or total protein in fish sampled from the alkaline water of Slapton Ley. The radio-tracking provided no evidence of adult perch and pike (Esox lucius) trying to seek refuge from the alkaline conditions, despite having access to adjacent parts of the lake with neutral pH. It seems likely that there are advantages (e.g. better foraging, less predation) of withstanding the high pH conditions that outweigh the benefit of moving into more pH neutral parts of the lake.