UNLABELLED: Circuit resistance training (CRT) performed three times weekly for 40-45 min each session increases muscular strength and both aerobic and anaerobic capacity in untrained individuals with chronic paraplegia. Whether similar CRT is also effective for conditioning of persons with chronic tetraplegia is unknown. In addition, protein supplementation (PS) before and immediately postexercise has been shown to enhance exercise adaptations. PURPOSE: This study aimed to investigate whether a modified 40-45 min CRT program will improve fitness attributes in individuals with tetraplegia and whether these changes are enhanced by PS. METHODS: Eleven individuals with chronic tetraplegia underwent 6 months of CRT performed three times per week. Six randomly assigned participants received PS (whey protein = 36-37 g) in split doses immediately before and after exercise sessions. Others consumed a matched protein dose 24 h postexercise. Measurements of one-repetition maximum (1-RM) strength for six different resistance exercises, arm peak oxygen consumption (VO2peak), and arm anaerobic power (Wingate) were obtained 3 months before (-3mo), at the beginning (0mo), 3 months into (3mo), and 6 months after (6mo) the beginning of CRT. RESULTS: One-repetition maximum increased by 8%-11% ± 6%-12% for each successive 3-month period (P ≤ 0.001-0.012), independent of PS group (P = 0.105). VO2peak increased significantly from 0mo to 6mo with immediate PS (35% ± 29%, P = 0.020) but failed to reach significance for delayed PS (15% ± 8%, P = 0.147). Power drop changes during the Wingate test were also only significant for the immediate PS (median difference 40W, P = 0.028) and not for delayed (10W, P = 0.500). CONCLUSION: CRT effectively increased muscular strength, aerobic capacity, and anaerobic fatigue resistance in persons with chronic tetraplegia. The latter two conditioning benefits were further enhanced by timely PS.