ABSTRACT.The respiratory responses to theophylline during normoxia and hypoxia were determined in 13 unanesthetized fetal sheep. Theophylline (plasma levels ∼ 111 &mgr;mol/L) increased the incidence of fetal breathing movements measured over 120 min from 37.7 ± 4.8% to 61.1 ± 5.7% (SEM) in normoxic fetuses. In isocapnic hypoxia (arterial O2tension ∼ 1.86 kPa), theophylline increased the incidence from 20.0 ± 6.3 to 52.0 ± 6.1%. Theophylline also resulted in an increase in the slope of inspiration during both normoxia and hypoxia. We conclude that adenosine modulates fetal respiratory drive during normoxia and hypoxia. (Pediatr Res28:83‐86, 1990)AbbreviationsECOG, electrocorticogram