To test the hypothesis that pulsed Doppler ultrasound spectral properties of bounded fluid jets related to orifice size, in vitro examinations were performed using a hydraulic simulator (standpipe, compliant receiving chamber, and variable round orifice). Orifices of 17.3, 7.9, 4.5 and 2.0 mm2 area resulted in flows of 4.0, 1.7, 1.1 and 0.3 L/min respectively. We interrogated the fluid jet at midchamber (MC) and at the chamber will impact site (IS), and the resulting Doppler shifts were displayed on a spectrum analyzer. For each orifice size, 36 independent observations were made at each interrogation site. At site MC, a characteristic low break frequency (K) was observed in an otherwise flat spectrum. The spectrum from site IS were peaked, with the peak described by center frequency (P), peak amplitude (A), and frequency at which the peak merged with the background (X). As orifice size increased from smallest to largest, monotonic changes in K (1020-440 Hz), P(330-750 Hz), A (8-28 db), and X (840-2660 Hz) were observed. Standard deviation about each mean ranged from 9 to 28%. Orifice shape, receiving chamber viscosity, and standpipe pressure significantly influenced the spectra. These data suggest that quantification of jet flow is possible in vitro using spectral analysis and pulsed Doppler ultrasound.