The nerve sheath of the thoracic ganglion of the house-fly (Musca domestica L.) is readily permeable to diazoxon but is impermeable to acetyl choline and acetyl thiocholine. Acetone and other lipoidal solvents, but not formaldehyde, were found to destroy the barrier to entry of the substrates, acetyl choline and acetyl thiocholine, and it is inferred that the barrier is lipoidal. At − 16°C., acetone does not destroy the cholinesterase of the ganglion.

Cholinesterase associated with the ganglion could be divided into three regions which showed different inhibition characteristics: (a) ‘ superficial ’ enzyme outside the nerve sheath, inhibited by 3·3 × 10−8M diazoxon; (b) ‘ peripheral ’ enzyme in the cellular region of the ganglion, inhibited by 3·3 × 10−8M and 3·3 × 10−9M diazoxon; (c) ‘ central ’ enzyme in the synaptic area, inhibited by not less than 3·3 × 10−7M diazoxon. Using inhibition of cholinesterase as an indicator of penetration, no difference in permeability was found between the ganglia of the susceptible and resistant strains. No difference was found in the inhibition of cholinesterase by diazoxon in the ganglia of susceptible and resistant strains.

It is concluded from this and previous work that if inhibition of cholinesterase of the nervous system is the cause of death there are no differences between the ganglia of the susceptible and resistant strains which affect resistance; and, furthermore, that inhibition of cholinesterase in the thoracic ganglion is unlikely to be the cause of death.