Endothelin-1 (ET-1) levels in the culture medium were considered to reflect the transcription of the ET-1 gene and the subsequent secretion of ET-1 from cultured cells. It has not been clarified how different ET-1 mRNA expression levels and immunoreactive (IR)-ET levels in the culture medium are in the cell culture system. We studied ET-1 mRNA expression levels and IR-ET levels in the medium of T98G glioblastoma cells treated with cytokines. T98G glioblastoma cells were cultured with cytokines (interferon-gamma 100 U/ml, tumor necrosis factor-alpha 20 ng/ml and interleukin-1beta 10 ng/ml) under normoxia or hypoxia (1% O(2)). Northern blot analysis showed that ET-1 mRNA expression levels were increased by tumor necrosis factor-alpha alone or a combination of tumor necrosis factor-alpha and interleukin-1beta, or three cytokines, and the increase was further enhanced under hypoxia. Particularly, relative expression levels of ET-1 mRNA were significantly higher under hypoxia than in normoxia in the treatment with a combination of three cytokines. IR-ET levels in the medium were increased by treatment with tumor necrosis factor-alpha, interleukin-1beta or a combination of tumor necrosis factor-alpha and interleukin-1beta, or three cytokines. In contrast to the mRNA expression levels, IR-ET levels in the medium of T98G cells treated with a combination of three cytokines were rather decreased under hypoxia compared with those in normoxia. These findings indicate that hypoxia induces ET-1 mRNA expression in the treatment of three cytokines, but IR-ET levels in the medium do not reflect this induction in T98G glioblastoma cells.