In the present study the dose-related effects of linear and cyclic somatostatin were compared on the self-stimulation rate of rats. Twenty micrograms of linear somatostatin administered intracerebroventricularly (icv.) markedly decreased the self-stimulation rate, while 5 micrograms and 10 micrograms was ineffective. Cyclic somatostatin in a dose of 1 microgram caused a transitory but not significant increase in the self-stimulation rate, which later returned to the control level. Five micrograms and 10 micrograms of the peptide decreased the self-stimulation rate. These results indicate that the cyclic somatostatin is more effective in inhibiting the self-stimulation rate than the linear one.