This study investigated the relationship among resistive load magnitude (DeltaR), the cortical evoked potential P(1) peak amplitude of the respiratory-related evoked potential (RREP), mouth pressure (Pm), esophageal pressure (Pes), transdiaphragmatic pressure (Pdi), and resistive load magnitude estimation (ME) in human subjects. The RREP, Pdi, Pes, Pm, and ME were recorded in response to three DeltaR values. The RREP was recorded from C(3) and C(4), referenced to the vertex C(Z). The group means of the Pdi, Pm, ME, and RREP P(1) amplitude increased with increases in the DeltaR. A log-log plot of the P(1) amplitudes showed a relationship with ME as did Pes, Pdi, and Pm. There were linear log-log relationships between C(Z)-C(3) P(1) amplitude, C(Z)-C(4) P(1) amplitude, and Pdi to ME. Pdi had a linear log-log relationship with C(Z)-C(3) and C(Z)-C(4). These results support the hypothesis that the estimated magnitude of the respiratory load is related to the P(1) amplitude of the RREP. Pm, Pes, and Pdi are mechanically related and correlated with the P(1) peak amplitude, suggesting that the mechanoreceptors mediating the P(1) peak of the RREP are activated by changes in mechanical forces related to the inspiratory pump.