Spontaneous and nitrosoguanidine (NG)-induced rate of reversions to glutamine independence was studied in cultured temperature-sensitive glutamine auxotrophs of Chinese hamster cells. In 3 experiments the spontaneous rate of reversions varied from 0.8-10(-6) to 3.84-10(-6) per cell per generation. A dependence of the yield of NG-induced back mutations upon the time interval between the mutagenic treatment and the transfer to selective conditions (glutamine deficient medium, 40 degrees C) was established. No induced revertants were detected when cells were transferred to selective conditions immediately after the treatment with NG. After 2--3 days cultivation in glutamine containing medium at 36 degrees C and the sunsequent transfer to selective conditions the frequency of induced reversions varied from 0.56-10(-4) to 10.55-10(-4) in different experiments; after 6 days -- from 0.05-10(-4) to 4.0-10(-4). In all cases where induction was detected, the difference, between the frequency of glutamine prototrophs in treated and control plates was significant. Glutamine independence proved to be stable after prolonged cultivation under non-selective conditions, the degree of prototrophy being greatly unequal in different clones. No differnce in this respect was detected between spontaneous and NG-induced revertants. The proposed system of reverse mutations can be used for studying diverse problems of somatic cell genetics.