In order to enhance the carotenoid content of potato tubers, transgenic potato plants have been produced expressing an Erwinia uredovora crtB gene encoding phytoene synthase, specifically in the tuber of Solanum tuberosum L. cultivar Desiree which normally produces tubers containing c. 5.6 microg carotenoid g(-1) DW and also in Solanum phureja L. cv. Mayan Gold which has a tuber carotenoid content of typically 20 microg carotenoid g(-1) DW. In developing tubers of transgenic crtB Desiree lines, carotenoid levels reached 35 microg carotenoid g(-1) DW and the balance of carotenoids changed radically compared with controls: beta-carotene levels in the transgenic tubers reached c. 11 microg g(-1) DW, whereas control tubers contained negligible amounts and lutein accumulated to a level 19-fold higher than empty-vector transformed controls. The crtB gene was also transformed into S. phureja (cv. Mayan Gold), again resulting in an increase in total carotenoid content to 78 microg carotenoid g(-1) DW in the most affected transgenic line. In these tubers, the major carotenoids were violaxanthin, lutein, antheraxanthin, and beta-carotene. No increases in expression levels of the major carotenoid biosynthetic genes could be detected in the transgenic tubers, despite the large increase in carotenoid accumulation. Microarray analysis was used to identify a number of genes that were consistently up- or down-regulated in transgenic crtB tubers compared with empty vector controls. The implications of these data from a nutritional standpoint and for further modifications of tuber carotenoid content are discussed.