The present study was aimed to compare antiproliferative effects of somatostatin (SS) and gonadotropin-releasing hormone analogs (GnRHa) on a fibroblast cell line. Proliferation index, cell count, viability of the cells and insulin-like growth factor-I (IGF-I) immunoreactivity were determined after treatment with either SS (100 mu M/ml), GnRHa (35 nM/ml) or SS and GnRHa of Balb-C 3T3 mouse fibroblasts. It was found that the proliferation index, cell count, viability and IGF-I immunoreactivity were not affected by GnRHa treatment as compared with no treatment (p > 0.05). Application of SS to the fibroblasts resulted in a significant reduction in proliferation index, cell count, and IGF-I immunoreactivity as compared with GnRHa treatment and no treatment, but it had no effect on cell viability. The labelling index in SS-treated cells was significantly reduced as compared with combined treatment with SS and GnRHa. In conclusion, a direct effect of GnRHa on fibroblast cells in culture could not be demonstrated. SS had direct inhibitory effects on cell proliferation possibly via inhibition of IGF-I effects without affecting cell viability.