Zero stability tests were performed on contrasting, commercially available, blood pressure-transducer systems. One system was based on a brand of disposable transducer. The others employed one brand of reusable transducer with and without samples of two different brands of compatible disposable domes. Drift was measured at atmospheric pressure over 3-hr periods. Drifts with the disposable transducers and with the bare reusable transducers were small, ranging from -2 to +2 mm Hg over 3 hr. However, the drifts of the reusable transducers with domes were significantly greater, ranging from -11 mm to +5 mm Hg. The disposable transducers did not drift significantly after the first half hour, although the reusable transducers with domes continued to drift. In addition, one brand of disposable dome produced inaccurate calibrations with the reusable transducer. The methodology of drift measurement and analysis should be practical and useful in other settings and with other brands of transducers. In general, the results indicate that periodic zeroing is still a clinically important procedure, and it is a worthwhile effort prior to treatment decisions based on pressure readings.