The question of amino acid requirements for DNA synthesis and cell division has been studied in Tetrahymena pyriformis by depriving cells of histidine and tryptophan at defined stages in the interdivision interval. Deprivation any time before DNA synthesis does not prevent the initiation of such synthesis but completely inhibits the following division and limits the increase in DNA, as measured microspectrophotometrically, to 20 per cent. H(3)-thymidine added to the medium is not incorporated during the 20 per cent increase. Deprivation after DNA synthesis is initiated does not prevent the continuation (to completion) of DNA synthesis, and cell division ensues. H(3)-thymidine added to the medium under these conditions is incorporated into macronuclear DNA. The data indicate that some amino acid-dependent event occurs, about the time of the beginning of the DNA synthesis period, which is not essential for initiation of DNA synthesis but which is essential for the maintenance of synthesis once it has begun. These results are further discussed in terms of enzymes required to convert thymidine (and possibly the other three deoxyribonucleosides) to the immediate precursor of DNA synthesis.