Abstract 1. 1. O 2 consumption, potassium and sodium concentrations were measured in kidney slices of the dogfish, Scyliorhinus canicula, incubated in normal dogfish medium containing urea and in media in which urea was replaced by thiourea (thiourea medium) or acetamide (acetamide medium). 2. 2. In thiourea medium O 2 consumption and K + concentration were decreased and Na + concentration was increased compared with the values obtained in urea medium. 3. 3. In acetamide medium O 22 consumption, K + and Na + concentrations were the same as in urea medium. 4. 4. These results support the idea that the high urea concentrations in elasmobranch tissues mainly exert a relatively unspecific osmotic effect rather than a specific biochemical effect. Thiourea appears to be toxic.