The aim of the present study is a comparative evaluation of the effects of pressure support ventilation (PSV) and intermittent mandatory ventilation (IMV) on diaphragmatic activity in rabbit model of neonate. The animals were divided into a PSV group and an IMV group. In the IMV group, spontaneous breathing and four kinds of IMV rate (5, 10, 15, and 20/min) were applied (Ventilator: Bear BP200, peak inspiratory pressure [PIP]: 12 cm H2O, inspiratory time: 0.6 s). In the PSV group, spontaneous breathing and four levels of PSV (3, 6, 9, and 12 cm H2O) were applied (Ventilator: VIP Bird, flow triggering). Airway pressure (Paw), flow (V), esophageal pressure (Pes), integrated diaphragmatic electromyogram (Edi), and arterial gas data were measured. Amplitudes of Pes and Edi were expressed as percentages (% Edi and % Pes) of the control value during spontaneous breathing to evaluate diaphragmatic activity. Lower IMV rates did not reduce diaphragmatic activity. Approximately half of diaphragmatic activity of control remained even at IMV 15/min. Diaphragmatic activity disappeared at IMV20/min. In contrast, PSV reduced Edi and Pes linearly according to support level. In conclusion, diaphragmatic activity could be reduced more gradually with PSV than IMV by altering ventilatory support level.