The mRNA content of the brain-specific metallothionein-III (MT-III) protein was measured by quantitative reverse transcription-polymerase chain reaction (RT-PCR) in two transformed glial cell lines: rat C6-glial and human U-373 MG cells. Low levels of MT-III mRNA were detected compared to a high expression of this mRNA in primary cultures of rat astrocytes. C6-glial cell lines stably transfected with a human 5-HT1D alpha or 5-HT1D beta receptor gene showed a decrease (87 to 93%) in basal [3H]thymidine incorporation, whereas their MT-III mRNA content was more than 30-fold increased compared to plasmid transfected C6-glial cells. The inverse proportion between mitogenic activity and MT-III mRNA content suggests that MT-III may act as a growth inhibitory factor in rat C6-glial cells.