On molecular sieve columns the neuropeptide head activator elutes at two distinct positions corresponding to apparent mol. wts of 700 and 1400 daltons. The low mol. wt component is stable only under high ionic conditions and represents the monomeric state of the head activator. Only this form is biologically active. The higher mol. wt component, which is reapidly formed under physiological conditions, is the dimeric head activator and is biologically inactive. We suggest that this dimerization is of biological relevance as a mechanism for inactivation of neuropeptides.