Manometrically measured peristaltic pressure amplitude displays a well-defined trough in the upper esophagus. Whereas this manometric "transition zone" (TZ) has been associated with striated-to-smooth muscle fiber transition, the underlying physiology of the TZ and its role in bolus transport are unclear. A computer model study of bolus retention in the TZ showed discoordinated distinct contraction waves above and below. Our AIM was to test the hypothesis that distinct upper/lower contraction waves above/below the manometric TZ are normal physiology, and to quantify space-time coordination between tone and bolus transport through the TZ. METHODS. 18 normal barium swallows analyzed in 6 subjects with concurrent 21-channel high-resolution manometry and digital fluoroscopy. From manometry, the TZ center (nadir pressure amplitude) and the upper/lower margins of the pressure trough were objectively quantified. Using fluoroscopy, we quantified space-time trajectories of the bolus tail and bolus tail pressures, and maximum intraluminal pressures proximal to the tail with their space-time trajectories. RESULTS. In every swallow the bolus tail followed distinct trajectories above/below the TZ, separated by a well-defined spatial "jump" that terminated an upper contraction wave and initiated a lower contraction wave (3.32 cm ± 1.63 , p = 0.0004). An "indentation wave" always formed within the TZ distal to the upper wave, increasing in amplitude until initiating the lower wave. As the upper contraction wave tail entered the TZ, it slowed and the tail pressure reduced rapidly, while indentation wave pressure increased to normal tail pressure values at the initiation of the lower wave. The TZ was a special zone of segmental contraction. CONCLUSIONS: The TZ is, physiologically, the transition from an upper contraction wave originating in the proximal striated esophagus, to a lower contraction wave that moves into the distal smooth-muscle esophagus. Complete bolus transport requires coordination of the upper/lower waves, and sufficient segmental squeeze to fully clear the bolus from the TZ during the transition period.