The addition of somatostatin to hippocampal synaptic plasma membrane (SPM) preparations in vitro decreased subsequent phosphorylation of specific protein bands. 10(-4)M somatostatin inhibited the phosphorylation of protein bands with apparent molecular weights between 10 000 and 20 000 daltons and, to a lesser extent, 48 000 daltons (B-50) and 52 000. Increasingly greater degrees of inhibition were seen in response to somatostatin-28 and [D-Trp8]somatostatin. Inhibition of B-50 protein phosphorylation in the presence of [D-Trp8]somatostatin was most prominent in SPM preparations from the hippocampus and amygdala, with lesser degrees of inhibition seen in the cortex and hypothalamus. Addition of [D-Trp8]somatostatin to an ammonium sulfate-precipitated fraction (ASP 55-80) from cortex only slightly inhibited endogenous B-50 phosphorylation. The injection of [D-Trp8] somatostatin intracerebroventricularly into rats did not induce excessive grooming behavior but resulted in barrel rotation. These results suggest that somatostatin and congeners affect SPM protein phosphorylation in a manner different from that of ACTH, presumably involving membrane sites that bind somatostatin.