Marcel-Millet, P, Ravier, G, and Groslambert, A. Effect of protective equipment on firefighters' external and internal workloads during a simulated rescue intervention. J Strength Cond Res XX(X): 000-000, 2020-Firefighter's protective equipment is one of the main factors increasing psychophysiological demand during rescue intervention. Workload quantification defines the amount of stress placed on an individual. This study aimed to test the relationships and compare different workload methods to discriminate 3 protective equipment conditions based on simulated rescue intervention. The protective equipment was (a) personal protective clothing (PPC); (b) PPC and a self-contained breathing apparatus (SCBA), including a cylinder, a full-face piece, and a breathing regulator; and (c) PPC and only the cylinder of the SCBA (SCBAc). Workload was determined using methods based on heart rate (HR), perceived exertion (sRPE), acceleration, and an indirect method based on postexercise HR variability. Differences between conditions were analyzed with repeated-measures analyses of variance. Relationships between workload methods were determined using Pearson's correlations. Statistical significance was set at p < 0.05. The HR-based and sRPE methods showed higher values in the SCBA and SCBAc conditions than the PPC condition (p < 0.0001); only the sRPE method had a higher workload for the SCBA condition than the SCBAc condition (p < 0.0001). Acceleration-based workloads were lower in the SCBA and SCBAc conditions than the PPC condition (p < 0.0001). The indirect method revealed no difference between the 3 conditions. Significant relationships were observed between the HR-based and sRPE methods (r = 0.544-0.738). Inverse significant correlations were observed between the HR-based and sRPE methods and the acceleration workload methods (r = -0.319 to -0.762). Although HR-based and sRPE methods might quantify the workloads, the sRPE method was more sensitive to discriminate between the SCBAc and SCBA conditions.