An economic reliable long-term recirculation anaesthesia system for laboratory fish is described. Anaesthesia of channel catfish ( Ictalurus punctatus Rafinesque) was induced within 60 s and was maintained for up to 40 min using tricaine methanesulphonate; recovery occurred within 30-60 s. Various surgical procedures were performed on the fish. No deaths were recorded. All water-quality parameters tested over 19 days use of the system remained stable except for total ammonia nitrogen and, by calculation, un-ionized ammonia which increased to a maximum of 0·23 mg/1.