Four groups of six animals each were exposed to simulated +Gz aerial combat maneuvers (SACM). The first group was exposed to 5 SACM/d for 1 d. The other three groups were exposed to 5 SACM/d three times per week for 1 week, 1 month, and 6 months, respectively. The SACM was 100 s long and contained two 5-s +9Gz peaks. A control group of four animals was exposed to 1 d of five 100-s +1Gz exposures. All animals were unanesthetized. The animals were necropsied and the hearts were examined macro- and microscopically within 48 h of their respective acceleration regimen. Eight transmural samples were taken from each heart for hydroxyproline analysis. Subendocardial hemorrhage (scale = 0-4) and myocyte damage (damaged cells per tissue section), respectively, were greatest in the control (2.08 and 0.87) and the 1-d (2.39 and 0.84) exposure groups, declining thereafter to the lowest values in the 6-month (0.33 and 0.06) group. Hydroxyproline concentration, as an indicator of possible scar-tissue development, showed a significant positive trend with increasing time at several sample sites on the left ventricular free wall. Heart rate (HR) and ventricular ectopy were highest during the early + Gz exposures and decreased with time thereafter. Seven additional swine were chronically instrumented with an arterial catheter for blood collection during the SACM to determine hematocrit and plasma levels of catecholamines, cortisol, and protein. The animals were exposed to the same 5 SACMs/d, 3 times/week for 4 months. Plasma norepinephrine (norepi), epinephrine (epi), and cortisol increased, respectively, from mean pre-G values of 0.9 ng/ml, 0.8 ng/ml, and 10.8 micrograms/dl to mean high values of 53.8 ng/ml, 58.1 ng/ml, and 34.0 micrograms/dl during 1-d and 1-week +Gz exposure; then declined to high values of 8.6 ng/ml, 4.3 ng/ml, and 18.8 micrograms/dl after 4 months of +Gz. The decline in macro- and microscopic myocardial damage during the approximate time frame of the decline in HR, arrhythmias, norepi, epi, and cortisol suggests a causative relationship. This decline also suggests that the injury observed during +Gz exposures is the result of the emotional stress of handling, restraint, and the unfamiliar environment of the centrifuge and +Gz, to which the animal becomes accustomed with time. However, the hydroxyproline and histologic data also suggest that myocardial scar-tissue may develop as a result of multiple +Gz exposures.