Gill and lung ventilatory frequencies at 20-23 degrees C were recorded in five different larval stages and in the adults of the bullfrog, Rana catesbeiana (n = 101, body mass 40 mg-90 g). Ventilatory frequencies in unanesthetized, unrestrained animals were determined (1) after normoxic acclimation, (2) during acute hypoxic and hyperoxic exposure, and (3) after brief but intense activity. Gill ventilation frequency (fG) under normoxic, resting conditions was 110-120 cycles X min-1 immediately after hatching, but fell to and remained at 40-50 cycles X min-1 for the remainder of larval development. Activity caused a sharp decrease in fG in newly hatched larvae, a sharp increase in larvae between stages IV-XIV, and no change in fG in all older larval states. Hypoxia increased fG in younger larvae up to developmental stage XIV, but had no effect upon fG in older larvae. Lung ventilation was rare in normoxic, resting larvae up to stage X. Thereafter until metamorphosis lung ventilation frequency (fL) was 2-6 breaths X h-1, with fL in adults being much higher at 1-3 breaths X min-1. Activity did not affect fL in any larval stage, but markedly increased fL in adults. Hypoxia had no significant effect on mean fL in larvae below stage XX. Mean values of fL increased during acute hypoxic exposure in most adults, but these changes were not significant. Collectively, these data indicate that progressive larval development is accompanied by a decline in reflex regulation of branchial ventilation frequency well before reabsorption of gills occurred. At the same time, respiratory responses are 'transferred' to the lung prior to metamorphosis and the attendant increasing dependence on air breathing.