The cellular action of growth factors, among them basic fibroblast growth factor (bFGF), is mediated by their interaction with a cell surface receptor, but the mechanism of transfer of mitogenic (or other) signals to the nucleus has not been identified. In this work, we show that bFGF is translocated to and accumulated in the nucleolus. Furthermore, the nucleolar localization of bFGF is correlated with a stimulation of transcription of ribosomal genes during G0----G1 transition induced by bFGF alone in adult bovine aortic endothelial cells (ABAE cells). Stimulation of ribosomal gene transcription is preceded by a significant increase of the major nonhistone nucleolar protein, nucleolin. In vitro, the growth factor has a direct effect on the enhancement of RNA polymerase I activity in isolated nuclei from quiescent sparse (G0) ABAE cells. The direct action of bFGF on the level of ribosomal gene transcription could correspond to an additional growth-signaling pathway, mediated by this growth factor.