Distribution and staining intensities of growth hormone releasing factor (GR (GRF) and somatostatin (SRIF) were examined in young (3 months of age) and old (24 months of age) male rats of Sprague-Dawley strain, using the PAP immunocytochemical procedure. Some animals of each age group were intraventricularly injected with colchicine to demonstrate immunoreactive neuronal perikarya. GRF-immunoreactive intensities of old rats were markedly reduced in the median eminence as compared with those of young rats. No remarkable difference could be detected between SRIF immunoreactivities in the young and old animals, since intensive SRIF immunoreactivities were found in the external layer of the median eminence of both groups of animals. Between two age groups injected with colchicine, we also found no difference in the distribution and staining intensities of immunoreactive perikarya of GRF and SRIF in the hypothalamus and also detected no significant difference in total neuron numbers of each peptide. These findings suggest that the synthesis and/or release of GRF in GRF-containing neurons are decreased, though GRF-containing neurons themselves remain alive and have the capacity to synthesize GRF.