Telomerase, responsible for telomere synthesis, is expressed in approximately 90% of human tumor cells but seldom in normal somatic cells. In this study, inhibition by carbocyclic oxetanocin G triphosphate (C. OXT-GTP) and its analogues was investigated in order to clarify the susceptibility of telomerase to various nucleotide analogues. C. OXT-GTP competitively inhibited telomerase activity with respect to dGTP However, C. OXT-GTP had a potent inhibitory effect on DNA polymerase alpha. It was examined whether the nucleoside (C. OXT-G) was able to alter telomere length in cultured human HL60 cells. Contrary to expectation, long-term treatment with 10 microM C. OXT-G was found to cause telomere lengthening.