The aim of this study is to evaluate the use of tetrazolium reductase (TR) activity as an indicator of myocardial viability in an isolated arrested pig heart biopsy model. Methyl Tetrazolium (MTT) is cleaved by an enzyme in the presence of coenzymes NAD, NADP. Cleavage yields a highly colored formazan product which is DMSO soluble. Efficient bioreduction of MTT has been investigated with heart biopsies. The relationship between MTT reduction and (1) oxygen consumption (r = 0.96, P < 0.001), (2) the sum of the adenine nucleotide levels (r = 0.87, P < 0.001) and (3) localization of coloration, has been established. The use of MTT in colorimetric assays offers high sensitivity. MTT reduction is a valid method. It is rapid and reproducible, and can be used as an indicator of myocardial viability. The MTT test has been used to rapidly compare the effect of different cardioplegic solutions (St Thomas and improved St Thomas) on hypothermic cardiac preservation. Significant differences have been established between the two solutions (P < 0.01).