A morphometric analysis of the corpus striatum of rats chronically treated with haloperidol was performed at the light and electron microscopic levels. Although the density of striatal neurons was unchanged in the haloperidol-treated group, there was a small increase in neuronal size (13%). This change in cell size was paralleled by a trend towards larger dendrite calibres occurring in the drug-treated animals. The distribution curve for axon terminal size indicated that 12% of the overall population was shifted from a range with a median size of 0.8 micron 2 to one with 1.6 micron 2 in the drug-treated group. This increase in size of some striatal terminals was accompanied by a concomitant increase in numbers of their associated synaptic vesicles, resulting in a similar density of vesicles for both control and drug-treated animals.