The heads of noenatal Wistar rats were irradiated with 200 rads daily from birth to the 10th day post-partum. Ten litters each containing 5 animals were killed at 30 days post-partum and their brains treated by the Golgi-Cox technique. The dendritic trees of 24 Purkinje cells were analysed using the quantitative technique of network analysis, and comparisons made between parameters obtained from 20 normal Purkinje cells. All dendritic trees in agranular irradiated cortex were markedly reduced in size (as indicated by total dendritic length and total number of segments) although mean path lengths were normal. Segment lengths were normal over proximal branches, but uniformly increased over distal branches. Abnormal appendages, called 'giant spines' were observed on many dendrites. They were often some 10 mum in length and their presence effectively reduced segment lengths, increased the frequency of trichotomy and deviated growth from the normal random terminal pattern so that long collateral branching topologies were formed. Nevertheless, trichotomy was uniformly reduced in those trees without 'giant spines' and the distribution of branching patterns suggested that growth had proceeded by random terminal dichotomy. These results demonstrate that the development of dendritic trees is retarded in the agranular irradiated cerebellum, where synaptogenesis is very greatly reduced below normal. The quantitative changes in segment lengths, size of trees, and trichotomy accord with those predicted by the filopodial synaptogenic hypothesis of dendritic growth formulated by Vaughn et al. 99, whilst the results of the topological analysis suggest that branching is established by a degree of non-random interaction between growing dendrites and their substrate. 'Claw-like' dendritic complexes within some Purkinje cell trees may have been induced by aberrent fibre bundles of few surviving granule cells.