A systematic sample of 2800 articles published between 1970 and 2000 in 14 major biomedical journals was analysed to assess the implementation of the ‘Three Rs’ (replacement, reduction and refinement) in biomedical research. During this period the total number of articles published annually by the journals more than doubled, but the proportion of studies using animals decreased by 30 per cent. There was a significant increase, from 21 per cent to 35 per cent, in the proportion of animal studies which made use of untreated euthanased animals as donors of biological materials, a gradual decrease in the number of chronic studies on animals, and a 50 per cent decrease in the average number of animals used per published paper. There was an improvement in the reporting of the specification of the animals' husbandry, conditions of care and environment. Parameters of importance for the evaluation of the scientific quality of the research and the welfare of the animals were generally poorly reported, but the proportion of papers with adequate information on most of the parameters analysed increased between 1970 and 2000.