It has been suggested that the initial phase of the ventilatory response to exercise is governed by a mechanism which responds to the increase in pulmonary blood flow (Q)--cardiodynamic hyperpnea. Because the initial change in stroke volume and Q is less in the supine (S) than in the upright (U) position at the start of exercise, we hypothesized that the increase in ventilation would also be less in the first 20 s (phase I) of S exercise. Ten normal subjects performed cycle ergometry in the U and S positions. Inspired ventilation (VI), O2 uptake (VO2), CO2 output (VCO2), corrected for changes in lung gas stores, and end-tidal O2 and CO2 tensions were measured breath by breath. Heart rate (HR) was determined beat by beat. The phase I ventilatory response was markedly different in the two positions. In the U position, VI increased abruptly by 81 +/- 8% (mean +/- SE) above base line. In the S position, the phase I response was significantly attenuated (P less than 0.001), the increase in VI being 50 +/- 6%. Similarly, the phase I VO2 and VO2/HR responses reflecting the initial increase in Q and stroke volume, were attenuated (P less than 0.001) in the S posture, compared with that for U; VO2 increased 49 +/- 5.3 and 113 +/- 14.7% in S and U, respectively, and VO2/HR increased 16 +/- 3.0 and 76 +/- 7.1% in the S and U, respectively. The increase in VI correlated well with the increase in VO2, (r = 0.80, P less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)