Growth hormone, prolactin and somatostatin-like immunoreactivities were demonstrated in the brains of larval, young adult (parasitic) and upstream migrant adult sea lampreys, Petromyzon marinus, by means of immunoperoxidase techniques. Growth hormone (GH) and prolactin (PRL) were observed within separate perikarya in the nucleus praeopticus, within fibers in the commissura praeinfundibularis, and in nerve endings within the neurohypophysis of larval and adult-stage lampreys. Cell bodies demonstrating immunoreactive growth hormone were more numerous than those reactive for prolactin. Unlike in the upstream migrant adult lamprey, no GH or PRL was demonstrated in the adenohypophysis of larval or parasitic lamprey. Somatostatin (SRIF)-like immunoreactive neurons were demonstrated in the nucleus commissurae praeinfundibularis, anterior and posterior pars ventralis hypothalami, pars dorsalis thalami, and the tegmentum motorium rhombencephali of larval, parasitic and upstream migrant adult lampreys. Many of the SRIF containing neurons within the hypothalamus were cerebrospinal fluid (CSF)-contacting cells. SRIF fibers were found throughout most of the brain predominating within the nucleus praeopticus, pars ventralis hypothalami, and the nucleus interpeduncularis. No SRIF immunoreactivity was found within the neurophyophysis. The possible functions of these peptides within the brain of the lamprey are discussed.