The aim of the present study as to investigate whether increased central hypervolemia induced by tail suspension (TS) in the rat is an appropriate model of cardiovascular deconditioning (CVD). First, the physiological relationship between central venous pressure (CVP) and extracellular fluid volume (ECFV) was studied. TS (20 degrees) increased CVP (5.8 +/- 0.7 vs. 2.8 +/- 0.8 mmHg; P < 0.01). After 24 h of TS, CVP had returned to control range while ECFV was reduced by 19%. CVP kinetics during 24 h of TS was not affected by either reduction (-20%) or augmentation (/35%) of the ECFV. The normalization of CVP is likely to be a consequence of ECFV reduction, which itself is reduced by increased urinary excretion of water and sodium. Second, recovery from TS was studied. Resumption of the horizontal position was shown to be associated with a significant increase of heart rate (HR) and a slight reduction of blood pressure (BP); there was an apparent delay between increased HR and reduced BP. This imbalance between HR and BP is compatible with CVD. A model of simulated orthostatism (SO) was developed to further investigate the responses of HR and BP. Interestingly, SO (90 degrees rotation) in the normal rat was associated with significant tachycardia and a slight increase of BP. This pattern remained stable for at least 3 h. In rats that were tail suspended for 48 h, episodes of hypotension and bradycardia (5 +/- 1 in 3 h) suggested a defect in adaptation to increased hydrostatic pressure. In conclusion, TS appears to be an appropriate model of CVD. Reduction process. Return to horizontal position in TS rats induced a tachycardia with minimal effects on BP; this pattern is close to that observed in humans assuming upright posture. SO in previously TS rats disclosed episodes of hypotension and bradycardia that deserve further investigation.