Under physiological conditions, the response of Xenopus Iaevis laryngeal muscel fibers to nerve stimulation is sexually differentiated: subthreshold potentials are common in males and rare in females. This sex difference in muscle fiber response is correlated with sex differences in vocal behavior. Three-dimensional reconstructions of the pre- and post-synaptic elements from male and female frogs were performed to determine whether there are any differences in synaptic structure which might account for the observed sex differences. These reconstructions were performed at the facilities of the National Center for Microscopy and Imaging Research using Synu software. Results of this analysis, combined with results using freeze-fracture indicated that there were no ultrastructural differences between male and female synapses that could account for the differences in synaptic strength. Based on physiological analysis, it was concluded that the observed differences were due to quantal differences in the amount of transmitter released: transmitter release is less in males. This work was recently completed and finally published in the Journal of Neuroscience (Tobias, Kelley and Ellisman, 1995).