Experiments were performed on conscious chronically instrumented rats maintained on tail suspension to determine the time course of changes in baroreceptor control of heart rate produced by this procedure. Pressor responses were elicited by bolus injections of graded doses of phenylephrine and sodium nitroprusside, permitting evaluation of the totality of the sigmoidal curve relating mean arterial pressure to heart rate. Compared with control rats maintained at 0 degrees, rats maintained at 20 degrees using tail suspension for 24 h showed a significant reduction in reflex gain (-3.9 +/- 0.1 vs. -5.8 +/- 0.3 beats/min; P < 0.001) together with a reduction in the upper (472 +/- 11 vs. 512 +/- 5 beats/min; P < 0.01) and lower (270 +/- 3 vs. 284 +/- 2 beats/min; P < 0.01) plateaus of the sigmoidal curve. In three groups of rats, the sigmoidal curve was studied successively after returning for 1, 24, and 48 h at 0 degrees. The observed change in the gain of the reflex returned toward control values after 48 h, whereas the other parameters characterizing the sigmoidal curve did not change significantly. Transient but significant modifications of heart rate (tachycardic response) after the immediate return to the horizontal position were observed. The study provides evidence that 1) a significant change of the totality of the sigmoidal curve characterizing baroreceptor control of heart rate occurs very early after tail suspension in rats and 2) the gain of the reflex is restored during the 48 h after release of tail suspension, whereas the other parameters characterizing the curve, particularly the plateaus, remain altered.