The effect of light on nucleotide modifications in the tRNA of cucumber (Cucumis sativus L. var. Guntur) cotyledons was studied by chromatographic, electrophoretic and immunological methods. The tRNA from light-grown tissue showed the absence of 2-methylguanosine and a decrease in the relative proportions of ribothymidine and cytokinin-active ribonucleosides when compared to those produced from dark-grown tissue. On the other hand, a significant amount of one species of 2'-O-methyldinulcleotide was observed in the tRNA of light-grown tissue which was not detected in the dark-grown tissue. Also, tRNA from light-grown tissue had higher levels of another species of 2'-O-methyldinucleotide. The results showed no difference in the amounts of other modified nucleosides in tRNA between tissues grown under the two conditions. 2'-O-Methyl-1-methyladenosine, a nucleotide modified both in the base and the ribose, apparently specific to plant tRNAs, has been found to be present in the RNA of both light- and dark-grown tissues. These results on the variation in modified nucleotides suggest that light has some role in nucleotide modification and, consequently, in cellular functions.