Advances in Comparative Physiology and Biochemistry, Volume 6, presents three papers linked by their relevance to comparative neurophysiology. The first paper on high-frequency hearing in mammals examines the sensitivity to, production, and behavioral utilization of high-frequency sound for a wide range of mammals from bats, rodents, whales, dolphins, and seals to the insectivores, primates, edentates, and carnivores. The second paper examines axonal flow and fast transport in nerves. Special attention is given to the differences in substrate and mechanism in slow and fast transport. The neuron is presented as a suitable cell type for the investigation of intracellular transport in general. The third paper on the secretory activity of neurons and related electrical activity presents a comparative assessment of all ""neurocrine"" activities, including those in the service of neuroendocrine coordination or of synaptic transmission of information. Special attention is given to the nature of vesicles containing the neurosecretions and transmitter substances and to the mechanisms of release. The electrical events accompanying such neurocrine activities are discussed.