We investigated the effects of aminophylline, isoproterenol, and neostigmine on decreased diaphragmatic contractility induced by hypercapnia. With the thorax open, the animal receiving mechanical ventilation, and a plaster cast around the abdomen, constant length and geometry of the diaphragm were maintained. Contractility was assessed by analysis of transdiaphragmatic pressure (Pdi) generated during supramaximal phrenic stimulation at different frequencies. Bilateral phrenectomy was performed to prevent spontaneous diaphragm movement. Hypercapnia (PaCO2, 85 mmHg) reduced Pdi by 10% at low and high frequencies of stimulation. Subsequently, aminophylline (20 mg/kg) restored Pdi to the control value at every frequency of stimulation (p less than 0.05), whereas neostigmine (0.25 and 1.0 mg) restored Pdi at low frequencies only (p less than 0.05). Isoproterenol did not improve Pdi at any frequency. Analysis of twitch characteristics revealed that hypercapnia reduced peak twitch amplitude by 17%, this being the underlying cause of the decrease in Pdi. Low and high doses of all 3 drugs significantly reversed this effect by improving peak twitch tension to values equal with or greater than control values (p less than 0.05). In addition, aminophylline (40 mg/kg) and neostigmine (0.25 and 1.0 mg) significantly increased time to peak tension of the twitch (p less than 0.05) and isoproterenol (5 and 20 micrograms/min) significantly decreased twitch half relaxation time (p less than 0.05). We conclude that aminophylline and neostigmine improve diaphragmatic contractility during hypercapnia by virtue of their potentiating effect on twitch amplitude, whereas isoproterenol does not increase contractility because the process underlying the decrease in twitch duration masks the effect of an improved twitch amplitude.