We have studied the expression and sequences of T cell receptor gamma and delta chain gene messages in intrathymic T cell precursors of mice irradiated with 600 rads. On day 7 after irradiation a high level of expression of gamma and delta chain messages was detected in thymocytes which were composed of a relatively high proportion of CD3+CD4-CD8- thymocytes. During further development of the precursors from day 7 to day 14 after irradiation, gamma and delta chain messages fell to low levels and alpha and beta mRNA levels increased. Nucleotide sequence analysis of 14 gamma and 10 delta chain complementary DNA (cDNA) in the thymocytes on day 7 revealed that there were 7 functional gamma chain transcripts composed of V gamma 2-J gamma 2-C gamma 2 or V gamma 1-J gamma 4-C gamma 4 gene segments, and only 1 functional delta chain transcript composed of the V delta M23-D delta 1-D delta 2-J delta 1-C delta gene segments. The repertoire of gamma chain and delta chain genes used in "radioresistant" intrathymic T cell precursors of adult mice appears to be limited.