We examined whether inspiratory muscle training (IMT) improved cycling time-trial performance and changed the relationship between limit work (W _lim) and limit time (T _lim), which is described by the parameters critical power (CP) and anaerobic work capacity (AWC). Eighteen male cyclists were assigned to either a pressure-threshold IMT or sham hypoxic-training placebo (PLC) group. Prior to and following a 6 week intervention subjects completed a 25-km cycling time-trial and three constant-power tests to establish the W _lim–T _lim relationship. Constant-power tests were prescribed to elicit exercise intolerance within 3–10 (Ex1), 10–20 (Ex2), and 20–30 (Ex3) min. Maximal inspiratory mouth pressure increased by (mean ± SD) 17.1 ± 12.2% following IMT (P < 0.01) and was accompanied by a 2.66 ± 2.51% improvement in 25-km time-trial performance (P < 0.05); there were no changes following PLC. Constant-power cycling endurance was unchanged following PLC, as was CP (pre vs. post: 249 ± 32 vs. 250 ± 32 W) and AWC (30.7 ± 12.7 vs. 30.1 ± 12.5 kJ). Following IMT Ex1 and Ex3 cycling endurance improved by 18.3 ± 15.1 and 15.3 ± 19.1% (P < 0.05), respectively, CP was unchanged (264 ± 62 vs. 263 ± 61 W), but AWC increased from 24.8 ± 5.6 to 29.0 ± 8.4 kJ (P < 0.05). In conclusion, these data provide novel evidence that improvements in constant-power and cycling time-trial performance following IMT in cyclists may be explained, in part, by an increase in AWC.