1. Three anatomically different types of lobster motor axons have been shown to possess different excitabilities under identical conditions.
2. The type, openers, which fired the longest trains of repetitive responses also had the lowest rheobase values, the longest utilization times, and the least accommodation.
3. A relationship between the time course of the local potential and the time course of the excitatory state has been disclosed.
4. The characterization of the excitatory state by simple two factor equations has been discussed and found to be only approximate since both the excitatory state and the local response tend to be somewhat oscillatory.