The effects of 8 wk of inspiratory resistive loading (30 min/d, 3 x/wk) on diaphragm mass, contractile properties, fatigue, and fiber dimensions were studied in 10 male Wistar rats. They were conditioned to breathe through a Hans-Rudolph device. Half of them had to overcome a moderate inspiratory resistance (MR; n = 5), whereas the others only had to overcome the small resistance (SR; n = 5) of the inspiratory valve of the device. Results were compared with control rats (C; n = 5) moving and breathing freely. At the end of training, animals submitted to MR and SR generated mean inspiratory pressures of -2.5 +/- 1.1 and -0.2 +/- 0.05 cm H2O, respectively. TI/Ttot was 0.60 +/- 0.06 and 0.57 +/- 0.05, respectively. Body and diaphragm weight were unaffected by loading. Little or no change in in vitro diaphragmatic twitch kinetics, force generation, and fatigability was found between the three groups. Nevertheless, cross-sectional area of all fiber types increased in the two loaded groups compared with control animals. This increase reached statistical significance for type I fibers in the MR group (846 +/- 74 microm2) compared with the C and SR groups (589 +/- 32 and 683 +/- 96 microm2, respectively, p < 0.05). For IIa fibers both training groups were significantly different from the control group (SR: 768 +/- 99 and MR: 790 +/- 108 versus C: 592 +/- 37 microm2, p < 0.05). A hypertrophy of type IIx/b fibers was seen in MR compared with control animals (C: 1,555 +/- 136, SR: 1,845 +/- 338, MR: 2,053 +/- 326 microm2, p < 0.05). No differences were present in fiber type proportions between the three groups. We conclude that in our training setup, 8 wk of intermittent long-term inspiratory loading stressed the diaphragm already with a small resistance resulting in hypertrophy of predominantly type IIa fibers. A higher resistance resulted in hypertrophy of all fiber types.