Theophylline was administered orally in a sustained-release preparation at a dose rate of 20 mg/kg bodyweight to six beagle dogs. A mean (+/- se) maximum plasma concentration of 12.2 +/- 0.65 micrograms/ml was obtained 5.17 +/- 0.70 hours after administration and it was calculated to have a mean residence time of 9.09 hours. When given at 40 mg/kg bodyweight twice daily for five days the theophylline preparation was well tolerated, but, the dogs developed a marked tachycardia and hyperpnoea which were attributed to the high plasma concentrations (> 30 micrograms/ml) reached during this dosage regimen.
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