It has been reported that injection of the antitumoral drug cis-dichlorodiammineplatinum at 10 days of life affects cerebellar development in rats. After a single dose of 5 micrograms/g of body weight, the formation of granule cells is decreased and the maturation of postmitotic neurons is slowed down. A substantial time after treatment, reduced cell packing density of the internal granule layer and atrophy of the molecular layer can be observed. In addition, there is degeneration of some Purkinje cells and Golgi neurons. In spite of all these alterations, the regular architecture of the cerebellar folia is retained in many places. In the present study, we used immunocytochemistry with an immune serum raised against glutamic acid decarboxylase to further characterize the cis-dichlorodiammineplatinum-induced alterations of GABAergic neurons. The aim was to examine cerebellar development and to test for factors controlling the settling of GABAergic circuits. At all post-treatment intervals, most of the Purkinje and Golgi neurons and molecular layer interneurons showed stronger anti-glutamic acid decarboxylase immunoreactivity than in controls; this may have been due to altered fixation because of cis-dichlorodiammineplatinum-induced damages to the blood vessels; but could also reflect cellular retention of the enzyme, maybe due to cis-dichlorodiammineplatinum-induced damage of the microtubular apparatus. After seven days, large roundish immunoreactive varicosities were present in the molecular layer adjacent to the Purkinje cell dendritic poles. These varicosities, which were not observed in control animals, may be terminals of Purkinje cell axon recurrent collaterals contributing to the supraganglionic plexus, whose abnormal development would compensate for the reduced inhibitory inputs from inhibitory interneurons and/or Purkinje cells, which degenerated at early post-treatment intervals. At later post-treatment intervals (15 and 21 days), there were also alterations in the pericellular basket at the Purkinje cell axon hillock, which was poorly developed in or absent from the majority of cells. The finding was confirmed by morphological observation of basket cells in Golgi-Cox preparation and immunocytochemistry with an antibody raised against 200,000 mol. wt phosphorylated neurofilaments. It is concluded that early changes in anti-glutamic acid decarboxylase immunoreactivity of neurons may be due to a direct interference of the drug with the cellular metabolic pathways. The late anomalies in the anti-glutamic acid decarboxylase immunoreactivity appear to be secondary to changes in the tissue cytoarchitecture rather than being primary cis-dichlorodiammineplatinum-induced lesions of the cells.