Fourteen preterm infants with apnea (body weight, 1052 +/- 44 g; gestational age, 30.2 +/- 0.5 wks; and postnatal age, 9.9 +/- 1.5 days) were studied in an effort to evaluate the effects of aminophylline on respiratory center output and respiratory reflex activity in the preterm infant with idiopathic apnea. This was done by using the airway occlusion technique. The infants were studied before and 48 h after aminophylline was begun as a treatment for apnea. Occlusion pressure, which reflects respiratory center output, was measured at 100 msec after occlusion started (P100) and at its maximum (Pmo). P100 increased from 2.4 +/- 0.2 to 3.1 +/- 0.2 cmH2O (P less than 0.005), and Pmo from 6.1 +/- 0.7 to 8.8 +/- 1.0 cmH2O (P less than 0.001) after aminophylline therapy was started. The % prolongation of inspiratory time during the occluded breaths, when compared to the unoccluded breaths increased from 26.2 +/- 10.6 to 55.8 +/- 12.5% (P less than 0.01). This reflects a significant increase in the strength of the Hering Breuer reflex. Effective elastance, a measure of respiratory load compensation, was significantly higher during aminophylline treatment. It increased from 1.09 +/- 0.14 to 1.33 +/- 0.14 cmH2O/ml (P less than 0.02).