1. The nature and location of the barrier system which partially protects the nervous system in insects has been investigated in the American cockroach by studying the fluxes of 14C-butanol, 14C-butyrate, 14C-butylamine and 3H-butyltrimethylammonium.

2. Disruption of the sheath covering the ganglia has little effect on the influx of butanol and butyrate, but increased that of butylamine and butyltrimethylammonium.

3. 2,4-Dinitrophenol slowed the influx of only those compounds whose metabolism it blocks.

4. Effluxes show a fast and a slow component. Disruption of the sheath increases the amount but not the rate of the fast component; it raises the rate of the slow component for butanol, butyrate and butyltrimethylammonium, but not always.

5. Nerve cords stained with methylene blue and freeze-sectioned show dye concentrated in the fat tissue surrounding the nerve cord, then in the neural lamella; a little penetrated the cell bodies of glia and neurones, but the neuropile was unstained.

6. These findings suggest that the external sheath plays some part in restricting cation influx; apart from this the barrier system is a function of the whole ganglion, except that the neuropile may enjoy special protection.

This research project was supported in part by N.I.H. grant 07804-06 and 10422-04.