The linear pressure fields generated by rectangular apertures at ultrasonic frequencies are considered, experimental results and theoretical predictions are presented both for focused and unfocused cases. Measurements were made in water using a circular transducer (38 mm in diameter) with various rectangular apertures in front of it. Focusing was achieved by the addition of perspex lenses. The aperture dimensions and focal lengths were chosen to be comparable with those encountered in medical ultrasound systems. The pressure field generated by the aperture was sampled using a 1-mm-diam bilaminar polyvinylidene difluoride (pvdf) hydrophone. Predictions for diffraction losses due to a rectangular transmitter and a circular receiver of finite size are also shown. The general behavior of the pressure field of a rectangular aperture is discussed and good agreement is shown between experimental measurements and theoretical predictions within the known limitations of both.