1. Aminophylline (cumulative concentrations of 0.036-3.60 mmol/l) produced a concentration-dependent increase in both tension developed (Td) and the maximum rate of rise of tension (dT/dt max) of the isolated hemidiaphragm of the rat both during direct single-pulse and subtetanic stimulation. 2. The repeated series of additions of aminophylline into the bathing medium (the second and the third series) produced even further, more pronounced potentiation of both Td and dT/dt max during subtetanic stimulation only, the potentiation being the strongest after the third series of additions of the drug ("antifatigue effect"). The antifatigue effect of aminophylline was much more pronounced than the antifatigue effect of the equimolar concentrations of caffeine. 3. The presence of intact beta 1-adrenergic receptors seems to be essential for the antifatigue action of aminophylline under our experimental conditions. 4. The antifatigue effect of aminophylline was not affected by reserpine or 6-OHDA pretreatment of rats. 5. In a Ca(2+)-free medium the stimulatory effect of aminophylline on Td and dT/dt max was abolished or depressed (single-pulse and subtetanic stimulation, respectively). After returning the muscle into the medium containing Ca2+, the effect of aminophylline was significantly potentiated during both types of the stimulation. 6. The antifatigue action of aminophylline was preserved even in the presence of nicardipine or its solvent in the bathing medium. 7. In the presence of heparin (which produced a significant depression of both Td and dT/dt max by itself during direct subtetanic stimulation) the stimulatory effects of aminophylline on Td and dT/dt max (the second and third series of additions) were significantly potentiated in comparison with the effects of the first series of additions of aminophylline (with no heparin in the bathing medium). 8. The dose-response curves for the effects of aminophylline in the presence of Ni2+ on Td and dT/dt max during direct single-pulse stimulation were significantly shifted to the right. Ni2+ by itself produced significant and dose-related depression of both Td and dT/dt max during single-pulse and subtetanic stimulation, the subtetanic stimulation being much more sensitive. The antifatigue effect of aminophylline during subtetanic stimulation was preserved in the presence of Ni2+. 9. Our results indicate the important role of the extracellular calcium and the involvement of intact beta 1-adrenergic receptors in the antifatigue action of aminophylline. Also, the potentiating effect of heparin on the antifatigue action of aminophylline is presumably due to the influx of extracellular calcium through L-type Ca2+ channels during subtetanic stimulation. Our results indicate the possibility of the presence of T-type calcium channels (which can be blocked by Ni2+) in the isolated hemidiaphragm of the rat, but they do not seem to be involved in the antifatigue action of aminophylline.