Cellular events which occur prior to cell collapse were examined in the root cortex of rice (Oryza sativa L.) during aerenchyma formation. Cell collapse started at a specific position in the mid cortex. These cells were distinct in shape from those located towards the periphery. Furthermore, cell collapse was preceded by acidification and the loss of plasma-membrane integrity in cells of the mid cortex. Subsequent death of neighboring cells followed a radial path. Microinjection of molecules of different sizes conjugated with fluorescein isothiocyanate (FITC) showed a molecular exclusion limit of between 9.3 and 19.6 kDa in the root cortex. Furthermore, large molecules, i.e. those around 9.3 kDa, were predominantly transferred in a radial direction, which coincided with the path of sequential cell death.