1. 1. The activity of single neurons in isolated abdominal cords of Procambarus clarki (Girard) has been studied. Evidence is presented that this activity is generated by mechanisms within the cord and frequently within individual neurons. The activity therefore may be called spontaneous. 2. 2. Such activity occurs in relatively few neurons, probably about forty ascending and twenty-four descending per pair of connectives. 3. 3. Most ascending units respond to d.c. stimulation of the cord only when current is applied to certain regions. These responses probably indicate the positions of pacemaker loci. About three-quarters of the units studied had such pacemaker loci in the caudal (sixth) ganglion. 4. 4. Variations in response of units to d.c. with changing geometry and strength of stimulating current suggest that in certain cases pacemaker loci may cover an extensive region (one to several connective-lengths) of the neuron.