The tapeworm Ligula intestinalis interferes with the pituitary-gonadal axis of its fish host so that parasitised fish are unable to reproduce. To investigate how this effect may be brought about, antibodies against salmon- and chicken-II type gonadotrophin-releasing hormones (sGnRH and cGnRH-II, respectively) have been used in an immunocytochemical investigation of the brain of roach, Rutilus rutilus. Brains were removed, without the pituitary gland, from fish obtained from the wild during the period from February to April, prior to spawning. The sGnRH-positive neuronal system comprised cell bodies and fibres located in the forebrain and midbrain areas. The cGnRH-II-positive neuronal system comprised cell bodies, tracts and fibres mainly located in the posterior hypothalamic lobes and medulla. Salmon-type GnRH-positive cell bodies were demonstrated in the midbrain tegmentum. In the forebrain, the olfactory lobes contained only a few short fibres immunoreactive for sGnRH whilst the floor of the ventral telencephalon was highly reactive for sGnRH. Long tracts of cGnRH-II-positive fibres were observed to course laterally within the hypothalamic lobes, whilst cGnRH-II-positive fibres originating in the medulla formed a continuous channel coursing to the spinal chord. GnRH-positive cells were not observed in the preoptic area using anti-sGnRH or anti-cGnRH-II antibodies. The presence of the tapeworm Ligula intestinalis had no discernible effect on the GnRH-positive cells or fibres. The interaction of L. intestinalis with the pituitary-gonadal system of the fish host is discussed.