In this study, the hydrodynamics in a horizontal stirred tank reactor is investigated. This type of reactor is used in industry for fast polycondensation processes. Overall circulation, poorly mixed zones and macro-mixing times are determined in scale models under turbulent (Re > 105) and laminar (Re <300) conditions using planar laser induced fluorescence. For both sets of conditions, the observed overall circulation is complex and changes when the length-to-diameter ratio is varied. Under laminar conditions, the flow appears to be chaotic. The poorly mixed zones change in location, number, and life span for different length-to-diameter ratios. Dimensionless macro-mixing times under turbulent conditions are correlated with parameter variations and show nonlinear relationships with fill ratio, length-to-diameter ratio, and Reynolds number. Under laminar conditions, macro-mixing times could not be determined unambiguously, but they are only 2.5 times larger than those found under turbulent conditions.