Dissociated neuronal cultures from several regions of the nervous system elaborate two populations of neurites which have features of axons and dendrites. The microtubule-associated protein tau appears to segregate to the axon in some of these culture systems, however it does not do so until after the development of morphological polarity. Despite this observation, tau very likely has some role in the development of polarity because in cultured cerebellar macroneurons taken from the rat embryonic day 15 primordial cerebellum, the inhibition of tau expression by antisense techniques resulted in the failure of a single minor neurite to elongate and form an axon-like neurite. Tau antisense given continuously for up to 72 h kept neurons locked in a stage with minor neurites only; however when released from the effects of the antisense they fully recovered. The administration of tau antisense after the development of polarity resulted in the loss of the axon-like neurite, while dendrite-like neurites continued to grow. Together these results suggest that dendritic differentiation in cerebellar macroneurons requires the prior elaboration of an axon-like structure.