Experiments were performed to evaluate the neuroendocrine and cardiovascular effects of osmotic stimulation in mice. Hypertonic saline (HS) was administered centrally or via the blood stream to conscious mice during measurement of blood pressure (BP), heart rate (HR) and plasma vasopressin (VP) and oxytocin (OT). A test of hypovolemia on VP secretion was also performed. Chronic carotid arterial cannulas were inserted for blood sampling, cardiovascular monitoring and vascular injections. Intracerebroventricular (ICV) cannulas were used for central injections. Vascular injection of HS (30 µl, 3.4 M NaCl) caused rapid and transient increases in plasma VP and OT. Plasma VP increased from 5.6 ± 0.9 to 10.0 ± 1.0 pg/ml, while plasma OT increased from 1.5 ± 0.6 to 8.6 ± 2.4 pg/ml at the earliest time point, immediately after ICV injection. ICV osmotic stimulation produced a rapid and sustained increase in plasma VP, with no change in OT. Plasma VP levels were increased from basal levels of 5.1 ± 1.5 to 13.1 ± 4.6, 11.4 ± 1.5, 12.6 ± 1.7 pg/ml at 0, 1 and 5 min after injection, respectively. ICV HS also increased plasma corticosterone. BP was increased by both vascular and central osmotic stimulation. Vascular HS increased BP immediately (Δ15.3 ± 1.7 mm Hg, 0 min) and transiently (Δ–3.9 ± 4.6 mm Hg, 5 min) while central HS produced a sustained increase in BP (Δ10 ± 1.4 and Δ9.8 ± 1.9 mm Hg, 0 and 5 min). Osmotic stimulation produced no significant changes in HR. Acute hemorrhage (∼10% decrease in blood volume) increased plasma VP (4.9 ± 1.0 vs. 8.4 ± 2.2 pg/ml). These results show the pattern of endocrine and cardiovascular responses to osmotic stimulation in conscious mice. They demonstrate that (1) there are extremely rapid changes in plasma VP and OT; (2) plasma OT is increased only after peripheral vascular hypertonic injection, and (3) central and peripheral osmotic stimulations are associated with pressor responses.