In this study, the hydrodynamics in horizontal stirred-tank reactors are investigated. The flow state, agitation power, and macromixing time have been determined experimentally. Two flow states, i.e., "slosh" and "ring", can be distinguished, with transition between the two states that shows hysteresis. The agitation power was determined by measuring the temperature increase upon mixing. The power number appears to be comparable to power numbers in unbaffled vertical vessels. A variation in fill ratio indicates that agitation energy dissipates uniformly throughout the reactor under laminar conditions. Under turbulent conditions, however, most energy is dissipated at the vessel wall. Using pulse-response measurements, macromixing times have been determined. The mixing times correlate with momentum input and liquid volume, thus indicating different hydrodynamics at large and small scales. A combination of mixing,, times and agitation power shows that, at small scale and intermediate fill ratios, the mixing is most energy-efficient.