Glutamate can both facilitate and inhibit dendrite outgrowth in vitro. The major effects of low levels of glutamate occur only on the dendrites (not the axon) of pyramidal neurons and may be important for modulating dendrite outgrowth during neuronal development in vivo. Cytoskeletal changes resulting from glutamate exposure must underlie these changes in dendrite outgrowth. In the present study, hippocampal neuron cultures were used to measure the outgrowth of both axons and immature dendrites in the presence or absence of 50 microM glutamate. Subsequently, neurons were extracted and fixed for immunofluorescent labeling of microtubules and rhodamine phalloidin labeling of microfilaments. Additionally, neurons were prepared for electron microscopy to examine dendritic microtubules at the ultrastructural level. Glutamate led to increased dendrite outgrowth in the short term (4 hr) and dendrite retraction in the long term (8 hr). After short-term glutamate exposures, no obvious morphological changes occur in either the microtubules or microfilaments. However, longer glutamate exposure causes a decrease in the number of microtubules in the distal region of retracting dendrites, and causes an increase in microtubule number in the dendritic shaft of both retracting and growing dendrites. Thus, the microtubule cytoskeleton may be involved in producing the changes in dendrite outgrowth caused by glutamate exposure.