Production of the heterologous protein, bovine aprotinin, in Saccharomyces cerevisiae was shown to affect the metabolism of the host cell to various extent depending on the strain genotype. Strains with different genotypes, industrial and laboroatory, respectively, were investigated. The maximal specific growth rate of the strains was reduced by 54% and 33%, respectively, upon the introduction of the gene encoding aprotinin. Growing the strains in sequential shake flask cultivations for 250 generations led to an increased maximal specific growth rate and a decrease in the yield of aprotinin as a result of the adaptation. Determination of the level of mRNA encoding aprotinin and the plasmid copy number pointed to different mechanisms responsible for the decline in aprotinin yield in the different strains.