1. Connectives of the ventral nerve cord of Manduca sexta consist of glia-ensheathed axons surrounded by a perineuriurn and an acellular neural lamella, which is greatly expanded on the dorsal surface. The glial cells are linked to one another by desmosomea and tight junctions; the latter also occur between adjacent perineurial cells. There no continuous circum-neural fat-body sheath.

2. A ten-fold change in the external potassium concentration results in a 43 mV change in the resting potential of de-sheathed connectives. Action potentials of such exposed axons are rapidly blocked in low-sodium or sodium-free saline and under these conditions neither calcium nor magnesium is able to maintain conduction. Spikes from de-sheathed preparations are rapidly abolished on exposure to 10-6M tetrodotoxin. These iindmgs indicate a conventional ionic basis of excitation for the axonal membrane of this insect.

3. Analyses of the haemolymph reveal a mean sodium concentration of 25.4 (s.E. ± 0.98) mM/1 and a mean potassium concentration of 25.1 (s.E. ± 1.74) mM/.1

4. Action potentials recorded from sheathed connectives are maintained for extended periods in sodium-free saline.

5. Exposure of most sheathed connectives to elevated potassium concentrations results in a two-stage depolarization. A rapid, single-stage, apparently extraneuronal potential change is, however, observed in some preparations.

6. These results on sheathed connectives indicate the presence of some peripheral barrier to the movements of sodium and potassium; the tight junctions between adjacent perineurial cells are considered to be possible sites of this restriction.

Senior Research Fellow of King's College, Cambridge.