Transformed callus cultures of Nicotiana tabacum were generated in which the SAM-1 gene from Arabidopsis thaliana encoding S-adenosylmethionine synthetase (SAM-S), under the control of the 35S promoter, had been integrated. The presence of the SAM-1 gene was detected in all tested transformants and the SAM-S activity correlated with the accumulation of SAM in the tobacco callus cultures. Three distinct phenotypic classes were identified among the transgenic cell lines in relation to growth of the cells, structure of the calli, and level of SAM. Transgene silencing was observed in several cultivated transgenic calli and this phenomenon was correlated directly with a low level of SAM-1 mRNA accompanied by a decrease of the SAM-S activity. The transgenic calli overexpressing the SAM-1 gene accumulated a high SAM level. The modifications in SAM-S activity were reflected in the pattern of secondary products present in the different cell lines, thereby demonstrating that the flux through the biosynthetic pathway of a plant secondary product can be modified by means of genetic engineering.