Platelet Derived Growth Factor (PDGF)-stimulated intracellular calcium signals were obtained from preneoplastic clones derived from C3H 10T1/2 mouse fibroblasts during a four-week transformation assay. By the end of the assay, when transformed colonies of cells were becoming apparent, the PDGF-stimulated calcium signal had reduced significantly. In addition, whole-cell patch-clamp electrophysiology measurements indicated a marked reduction in expression of T-type calcium channels. These observations demonstrate that PDGF-induced calcium metabolism changes as initiated cells progress to the transformed phenotype.