We studied the relationship between inspiratory muscle activity and lung volume in 5 normal subjects in whom hyperinflation to 78-83% VC was induced with exgernal expiratory resistances. While breathing at this high lung volume the most negative pleural pressure (Ppl) during inspiration was -23.4 +/- 2.3 cm H2O (mean +/- 1 SE), whereas the maximum expiratory Ppl was -4.2 +/- 1.6 in four and +31 cm H2O in one subject. Using relaxation pressure-volume curves of the chest wall, we reasoned that in the 4 subjects inspiratory muscles showed a substantial persistence of activity throughout expiration. The minimum inspiratory muscle force (Pmus) during expiration was 35.9 +/- 8.4% of the peak inspiratory Pmus. Similarly, the work of the inspiratory muscles in expiration was 57.8 +/- 9.5% of the work during inspiration. In all 5 subjects the diaphragm relaxed almost completely in expiration, as evidenced by the transdiaphragmatic pressure (Pdi), which fell during expiration to 10.0 +/- 4.1% of the peak inspiratory Pdi. Inspiratory intercostal and scalene electromyographic recordings, obtained in 3 subjects, demonstrated substantial activity in expiration. We conclude that during external, resistive, expiratory loading the volume of hyperinflation is influenced by persistent activity of inspiratory muscles in expiration, and that this is due largely to the inspiratory intercostal and accessory muscles rather than the diaphragm.