An optical measuring device with electrical timing unit was designed to identify the aspiration of inadequate volumes of samples into AutoAnalyzers. The device measures the duration of transit of solutions past a fixed point in the sample line. Considerable differences in transit times are found under normal operating conditions when standard Technicon components are used. The variation in transit times is greater between sampler modules than when the same module is used. Documentation of duration of sample aspiration in publications describing AutoAnalyzer methods would facilitate comparison of results between laboratories. A modification of the Technicon Model II sampler unit is described which markedly reduces the variation in sample aspiration.