The effects of low level phenobarbital administration (18 postcoitus to 21 days postnatal) on Purkinje cell growth and remodeling were studied from 3 to 20 weeks postnatal). The Purkinje cell dendritic trees were analyzed both metrically and topologically using the method of vertex analysis. The total segment length, mean terminal path length, and mean vertex path length were reduced in the treated cells. The pattern of segment frequency as related to equivalent orders was abnormal in the treated cells. The Va/Vb vertex ratios and the levels of trichotomy indicated that the treated cells underwent nonrandom remodeling, unlike the control cells which exhibited dichotomous, random terminal branching. These observations confirm that phenobarbital produces distinct long-term morphologic alterations in Purkinje cells.