Fines retention is a combination of fines deposition on fibers and fines flocculation, followed by entrapment of fines floes in a forming sheet. In the laboratory, fines flocculation is often studied in a dynamic drainage jar (DDJ) to mimic the hydrodynamic shear on a paper machine. However, the shear in a DDJ is very different from the shear on a machine. A flow geometry that might approximate shear in a headbox of a paper machine better is high Reynolds number flow through a tube because many headboxes contain a series of parallel pipes. We studied the deposition of fines and the flocculation of fines in a flow loop, with flow velocities on the order of a few meters per sceond, using a poly(ethylene oxide)-cofactor retention aid system. We found that fines deposition and flocculation follow the predictions of kinetic theories of Langmuir and Smoluchowski rather well despite the fact that fines are highly polydisperse. Fines were found to be flocculated even in the absence of a retention aid probably because of mechanical entanglements of fibrillar fines. Adding retention aids resulted in further aggregation. The detachment and floe breakup rates were found to be rather high, and extrapolation to papermaking conditions leads to the conclusion that fines deposition and flocculation are negligible in a headbox, at least for the retention aid system considered. This contradicts findings from DDJ experiments, which usually show appreciable fines retention. Perhaps a flow loop better represents flow conditions in a headbox, and a DDJ better represents flow conditions during drainage and formation.