The aim of the present study was to examine the fine structural changes in the superior cervical ganglion of adult rats following baclofen treatment. On the addition of this GABAB receptor agonist there was a transient increase in the number of coated vesicles and pits in the dendrites of principal sympathetic neurons. As compared to the control ganglia, we found a significantly higher number of synapses which had coated pits attached to the postsynaptic densities. In interpreting these observations it is suggested that the GABAB receptors play some role in mediating the synaptogenetic action of GABA, although the formation of vacant postsynaptic densities and spines was not seen after baclofen treatment.