We have investigated the optimal thermal conditions of a Fleisch no. 4 pneumotachograph (PT) necessary for recording maximal forced expiratory maneuvers (MFEM). Our PT assembly was tested with a computer-driven pump and found to be linear up to 14 l/s. Thermocouples (TC) were placed in a peripheral, mid, and a central capillary of the PT. Stable temperature control and consistent PT calibration were best obtained by proportional thermostatic control via the peripheral TC. When the PT was heated to 35 degrees C or above, expirations from either the pump (air at 33 degrees C saturated) or a subject cooled the PT, thus affecting its response. With the PT heated to 30 degrees C, repeated blows caused little change in PT temperature, with no evidence of condensation, thus indicating optimal thermal conditions for recording MFEM.