The role of cytosolic free Ca2+ ([Ca2+]i) in the induction of the immediate early gene c-fos by TRH or by phorbol 12-myristate 13-acetate (PMA) was studied in the clonal pituitary cell line GH4C1. It was found that c-fos mRNA levels were rapidly and transiently increased by TRH at physiological concentrations (1–100 nm). The effect of TRH was dependent on a rise in [Ca2+]i, and TRH stimulation of Ca2+ influx was essential for c-fos induction. Cell depolarization with K+, which produces a [Ca2+]i rise by soliciting Ca2+ influx via voltage-gated Ca2+ channels, was insufficient to induce c-fos. Blockade or downregulation of protein kinase C (PKC) strongly attenuated TRH stimulation of c-fos expression. Direct stimulation of PKC by PMA raised c-fos mRNA levels, but only under conditions permitting Ca2+ influx. We conclude that TRH induces c-fos mRNA by a mechanism dependent on PKC activation and on Ca2+ influx. The essential role of Ca2+ influx for PMA stimulation of c-fos mRNA suggests a novel pathway linking PKC stimulation to early gene expression.