The choice of antibacterial drugs for the treatment of bacterial diseases in farmed salmonids changed dramatically during the period 1980-1994. In terms of treatment doses, oxytetracycline chloride was the most frequently prescribed antibacterial drug during the periods 1980-1983 and 1985-1986. In 1984, prescriptions changed in favour of furazolidone and trimethoprim/ sulphadiazine (1:5). Oxolinic acid was introduced for use in farmed fish in Norway in 1987, and immediately became the drug of choice, comprising 36% and 50% of the prescribed treatment doses in 1987 and 1988, respectively. In 1989, flumequine was temporarily approved for use in farmed salmonids, and during the period 1989-1994 antibacterial drug therapy in farmed salmonids acquired the character of "mono-therapy' with the quinolones flumequine and oxolinic acid. This rapid change-over in the choice of drug may partly be explained by the development of bacterial drug resistance in farmed salmonids, both to oxytetracycline and trimethoprim/sulphadiazine. The prescribing of furazolidone declined to zero during the study period. The morbidity caused by bacterial infections was defined as the number of treatment doses of antibacterial drugs per kg biomass of farmed salmonids per year. It was estimated that during the period 1988-1995, an average of 39% (mean value) of farmed salmon received, in theory, an antibacterial cure once each year. In comparison, the corresponding figure for the period 1981-1988 was 60%. However, in 1993 this figure fell to 13%, and declined even further in 1994 to 2.3%. The practice of on-farm mixing of medicated feed, using prescribed raw materials (pure drug substances) or premix formulations, declined significantly during the period 1992-1994. This was due to the introduction, in 1992, of new regulations on the prescribing of drugs to farmed fish.