The pressure-time index (PTI = Pmouth/Pi max x Ti/Ttot) has been validated by Ramonatxo (J. Appl. Physiol. 78 (1995) 646 and by Jabour (Am. Rev. Respir. Dis. 144 (1991) 531 as a noninvasive tool for the assessment of inspiratory muscles load. Nobody until now has evaluated the correlation between the PTI and diaphragmatic activity. Further, the PTI has not been compared with another measures of respiratory muscle load such as the transdiaphragmatic pressure index or TTdi. The purpose of our study was to test the hypothesis that the PTI measured at the mouth (PTIm) is a noninvasive reflection of TTdf and electromyographic activity of the diaphragm (EMGdf). We studied 6 patients with COPD and 5 normal individuals at rest and during a CO2 rebreathing trial and simultaneously measured PTIm, TTdi and EMGdi. The curves of PTIm and EMGdi follows the same trend during the CO2 rebreathing trial with strong and significant correlation between these parameters (r = 0.89 P < 0.05 and r = 0.82 P < 0.05 for PaCO2 of 45 and 53 mmHg respectively). We conclude that PTIm measured as Pmouth/Pi max x Ti/Ttot is an adequate noninvasive method that reflect not only the diaphragmatic activity but also the inspiratory muscles load.