Stevenson AT, Scott JPR, Chiesa S, Sin D, Coates G, Bagshaw M, Harridge S. Blood pressure, vascular resistance, and +G_z tolerance during repeated +G_z exposures. Aviat Space Environ Med 2014; 85:536–42. Background: Cardiovascular reflexes that regulate blood pressure (BP) adapt during repeated exposure to +G_z acceleration separated by short (< 20 s) breaks, but whether this effect is preserved with longer intervals remains unknown. Methods: There were 17 subjects who completed 5 repeated gradual onset (0.1 G · s−1) runs (GOR1–5) to 60° peripheral light loss (PLL) on a human centrifuge, separated by 2 min. Heart rate (HR) and BP were measured before and during each GOR and noninvasive estimation of cardiac output (CO) was used to calculate total peripheral resistance (TPR). Results: Mean resting (± SE) systolic BP (138 ± 4 vs. 128 ± 3 mmHg) and TPR (13.9 ± 1.2 vs. 12.7 ± 1.0 mmHg · l−1 · min−1) were elevated after GOR1 and remained elevated thereafter. Compared with that before GOR1, resting HR was decreased (5–6 bpm) before GOR2–5. Resting CO decreased from 8.3 ± 0.6 L · min−1 before GOR1 to a nadir of 7.2 ± 0.5 L · min−1 before GOR4 and GOR5. The change in HR under increased +G_z decreased from +39 ± 3 bpm during GOR1 to +31 ± 3 bpm during GOR4 and GOR5, but the decrease in eye level BP under +G_z was unaffected. RGT did not change across the five GORs. Conclusions: Increased resting BP and TPR following a single +G_z exposure suggest alterations in the cardiovascular system expected to confer improved tolerance during subsequent exposures. However, these changes were insufficient to improve +G_z tolerance measured during repeated GORs separated by 2 min.