The hips and lower extremities of four complete paraplegic male subjects (T-6, T-7, T-8, and T-11) were stimulated with functional neuromuscular stimulation (FNS) via transcutaneous intramuscular electrodes (20 mA, 0-150 pulse width, and 20 Hz). Cardiopulmonary (CP) and/or cardiovascular (CV) responses were measured during maximal (seated) arm ergometry (AE), FNS, and FNS + AE. Subjects' lower extremities were stimulated with a 2 s walking cycle via a microprocessor computer. Data were collected with a SensorMedic MMC Horizon (VO2 and VCO2 at STPD and VE at BPTS). The mean MET level (1 MET = 3.5 ml O2/kg/min) during FNS was 4.8. Mean METS during FNS + AE was 10.3 and mean METS for AE was 7.2. Mean lactic acid (LA) after FNS, AE, and FNS + AE was 73 mg percent, 77 mg percent and 115 mg percent respectively. Respiratory exchange ratio (RER) (VCO2/VO2) was greater than 1.2 during the first 2 to 5 min of FNS but decreased to less than 1.0 during the second 5 min of FNS. Steady state VO2 and RER less than 1.0 indicated a FNS transition from anaerobic to aerobic metabolism. Subject T-11 had CV limitations during FNS and FNS + AE due to excessive LA from FNS (115 mg percent). Ventilatory (VE) responses during AE, FNS, and FNS + AE were consistent with VO2; and mean maximal VE and VO2 for subjects T-6, T-7, and T-8 during FNS + AE was greater than 90 percent of that observed in sedentary normals. The aerobic and anaerobic capacities of paraplegic subjects is primarily limited by available muscle mass rather than impaired CV or CP function.