OBJECTIVE To evaluate the induction of the proto-oncogene c-fos in cultured human ciliary muscle cells by prostaglandin F2 alpha (PGF2 alpha) by 17-phenyltrinor-PGF 2 alpha. METHODS Human ciliary muscle cells were grown to confluency in monolayer cell culture, placed in medium containing 1% fetal bovine serum for 5 days, treated by addition of PGF2 alpha or the trinor derivative, fixed, and then immunocytochemically stained using an antibody to c-Fos, the protein product of the translation of c-fos. RESULTS After treatment with either agonist, the mean induction score (proportion of brightly immunostained nuclei) increased to a maximal level during the first hour and returned to basal levels 4 to 8 hours after treatment. Increasing the agonist concentration increased the maximal level, but had no effect on the time course of the response. The dose responses after 1 hour of treatment with PGF2 alpha or 17-phenyltrinor-PGF2 alpha increased similarly between 1.6 x 10(-9) M and 2 x 10(-7) M. When treated with 1 x 10(-6) M of either agonist, however, the induction was half that obtained at 2 x 10(-7) M. CONCLUSIONS Exposure of ciliary smooth muscle cells to either PGF2 alpha or 17-phenyltrinor-PGF2 alpha induces an immediate early gene expression response that is similar to c-Fos induction in other cell systems. These results establish the basis for future investigations evaluating the potential role of c-fos induction in mediating the effects of PGF2 alpha on uveoscleral outflow.