Lower body negative pressure (LBNP) and lower body positive pressure (LBPP) are two opposite environments for the lower abdomen and lower extremities. We examined RR interval (RRI), blood pressure, forearm blood flow, cardiac output, plasma atrial natriuretic factor, and plasma catecholamines during sequentially administered LBNP and LBPP in order to define the cardiovascular and neuroendocrine reciprocity of these models of orthostasis (LBNP) and anti-orthostasis (LBPP). Six healthy young men were studied with a ramped onset of LBNP followed by a similarly ramped LBPP, spaced between periods of data acquisition during rest. Forearm blood flow decreased during LBNP and increased during LBPP. However, LBNP and LBPP did not produce reciprocal changes in cardiac output. No reciprocal changes were found in plasma catecholamines and plasma atrial natriuretic factor. The variability of RRI and systolic/diastolic blood pressures were analyzed within two bandwidths of interest: the low bandwidth, corresponding to the frequency of baroreflex-mediated variability (0.04 to 0.12 Hz), and the high bandwidth, representative of parasympathetically mediated variability at the frequency of respiration (0.22 to 0.28 Hz). The ratio of high to low bandwidth RRI variability was reciprocally diminished during LBNP and increased during LBPP, reflecting a relative decrease in the level of parasympathetic tone during LBNP and an increase during LBPP. In addition, the variability of the DBP within the low baroreflex bandwidth increased during LBNP and decreased during LBPP. The phase lag of RRI variability in relation to BP variability did not change reciprocally.(ABSTRACT TRUNCATED AT 250 WORDS)