The short-term effects of muscle relaxation with pancuronium bromide on arterial blood gas values, heart rate, and arterial blood pressure were studied in 49 neonates receiving mechanical ventilation. After pancuronium administration, mean PaO2 increased from 61.9 +/- 30.0 to 80.9 +/- 52.8 mm Hg (P = 0.006), mean PaCO2 decreased from 40.1 +/- 13.0 to 37.5 +/- 13.3 mm Hg (P = 0.03), and mean heart rate rose from 146.2 +/- 24.9 to 161.0 +/- 20.5 beats/min (P less than 0.001). Arterial blood pressure did not change significantly. When results were analyzed by diagnosis (meconium aspiration syndrome, hyaline membrane disease, pneumonia), only infants with meconium aspiration syndrome had a significant improvement in oxygenation (P = 0.008). Six of 18 patients with hyaline membrane disease responded to muscle relaxation, with a decrease in PaO2 of 10 mm Hg or more. No significant correlation was found between change in PaO2 and birth weight, gestational age, postnatal age, or change in heart rate. A weak correlation (r = -0.37, P less than 0.006) was observed between change in PaO2 and change in PaCO2. Although the reasons for the divergence in response in different groups of patients are not entirely clear, the risk of deterioration with pancuronium therapy should be kept in mind, particularly if treatment in an infant with hyaline membrane disease is being considered.