Intermittent ultrasound transmission during contrast echocardiography, so-called transient response imaging (TRI), amplifies contrast intensity. This effect of TRI is attributed to decreased microbubble destruction by reduced exposure time to ultrasound energy. The present study examined the hypothesis that the signal amplification produced by TRI is related to the baseline intensity present in the image and the velocity of flow. We performed second harmonic (2.5/5.0 MHz) imaging during both continuous (frame rate 55 Hz) and electrocardiogram-triggered TRI mode. Contrast images produced by perfluorohexane microbubbles (AF0150) in a steady flow model were obtained every minute throughout the decay phase at transit velocities of 8.1, 6.2, 3.4, 1.9, and 0.7 cm/sec. The decay of videointensity over time could be fitted to a sigmoid curve for both imaging modes with r > 0.99 for individual velocities. The intensity with TRI was greater than that with continuous imaging (CI) at any time and velocity. The mean increase in intensity between modes throughout decay was 8.2 ± 3.7, 12.8 ± 4.2, 25.7 ± 5.8, 49.5 ± 8.0, and 64.0 ± 14.4 gray levels for the respective velocity levels studied (p < 0.0001). Although varying with baseline intensity at early and late phases, the TRI amplification plateaued during middecay, and within the intensity range of 16 to 143 gray levels for CI and 67 to 186 gray levels for TRI, it showed no overlap among the different velocity levels. Thus the ability of TRI to enhance contrast opacification is much greater at low flow velocities, which has implications regarding the mechanism of TRI effect and preferential visualization of intramyocardial coronary arteries by this agent. Although this effect was influenced by the baseline intensity, it was relatively constant for each velocity level within an optimal intensity range during middecay, providing the basis for flow velocity measurement by contrast echo. (Am Heart J 1997;134:1066-74.)