This study examined the effects of dehydration and rehydration with water on Fos and FosB staining in the brainstem of rats. Male rats were water deprived for 48 h (Dehyd, n = 7) or 46 h followed by 2 h access to water (Rehyd, n = 7). Controls had ad libitum access to water (Con, n = 9). Brainstems were stained for Fos and FosB/ΔFosB using commercially available antibodies. In the nucleus of the solitary tract (NTS), the number of Fos stained neurons was significantly increased by dehydration and increased further following rehydration (Con 5 ± 1; Dehyd 22 ± 1; Rehyd 48 ± 5). The average number of Fos-positive cells in the parabrachial nucleus (PBN) was significantly increased only by rehydration (Con 12 ± 2; Dehyd 6 ± 2; Rehyd 51 ± 4). The area postrema (AP) showed significant increases in Fos staining after dehydration and rehydration (Fos: Con 4 ± 1; Dehyd 28 ± 3; Rehyd 24 ± 3). In the rostral ventrolateral medulla (RVL), Fos staining significantly increased after dehydration and this effect was reduced by rehydration (Con 3 ± 1; Dehyd 21 ± 2; Rehyd 12 ± 1). In contrast, Fos staining in the caudal ventrolateral medulla (CVL) was not significantly influenced following either dehydration or rehydration with water (Con 4 ± 1; Dehyd 4 ± 1; Rehyd 5 ± 1). FosB/ΔFosB staining in the NTS, AP, and RVL was comparably increased by dehydration and rehydration. In the PBN and CVL, FosB/ΔFosB staining was not affected by the treatments. Dehydration and rehydration have regionally specific effects on Fos and FosB/ΔFosB staining in the brainstem.