The Fos family comprises Fos and several subtypes of Fos-related proteins (Fras) such as FosB, Fra-1, Fra-2, DeltaFosB, and chronic Fras. Changes in the expression of Fos family proteins with time are not well elucidated, particularly during chronic stimulation. In the present experiments, we investigated quantitatively the time course changes in Fos, FosB and Fras immunoreactivity in the magnocellular neurons of the supraoptic nucleus (SON) during acute and chronic osmotic stimulation. A small number of Fos- and FosB-positive neurons were observed in the SON of control rats, while many Fras-positive neurons were seen in control animals. Significant increases in the numbers of Fos-, FosB-, and Fras-positive neurons were observed 2 h after acute osmotic stimulation by intraperitoneal (i.p.) injection of 3% NaCl solution. Although the number of Fos-positive neurons returned to the control level 4 h after i.p. injection, a significant number of FosB- and Fras-positive neurons were still observed 8 h after i.p. injection. During chronic osmotic stimulation by giving 2% NaCl solution for 2 and 5 days, a large number of Fos-positive neurons were observed, but the cessation of chronic osmotic stimulation by normal water drinking immediately decreased the number of Fos-positive neurons to the control level within 2 h. The number of FosB-positive neurons was increased with period of chronic osmotic stimulation, and a significant number were observed 2-8 h after the cessation of the stimulation. The number of Fras-positive neurons was also significantly higher during chronic osmotic stimulation, and this number was significantly high 2-8 h after the cessation of the stimulation. RT-PCR analysis demonstrated the persistent expression of c-fos mRNA in the SON during chronic osmotic stimulation. These results suggest that c-fos mRNA and Fos protein are constitutively elevated during chronic osmotic stimulation and the time course changes in Fos are different from those seen in FosB and Fras.