Immunoreactive material is localized in the rat pineal gland using certain antisera generated against synthetic LHRH. Our studies demonstrate that the behavior of a pineal substance with respect to antibodies generated against different conjugates of synthetic LHRH indicates that the immunostained material in the pineal may have a different structure or antigenic determinants different from that of the LHRH in the hypothalamus. A considerable number of analogs of LHRH have been synthesized. We have generated antisera against these analogs and cross-reactivity with LHRH was examined by radio immunoassay procedures and immunocytochemical characteristics were studied by comparing the staining in terminals of the median eminence and pineal. The results focus attention on the N-terminal portion of the LHRH molecule. The intense research presently underway to find synthetic analogs of LHRH with anti-gonadotrophic property attests to its significance in reproductive biology and in research on means of fertility control.