The hypothalamus of the teleost Poecilia latipinna was investigated using electron microscopy and the Falck-Hillarp fluorescence technique. Two ultrastructurally distinct types of neuron were observed in the nucleus preopticus (NPO). The large type 1 neurons could be distinguished from the smaller type 2 by the appearance (although not size) of the dense-cored vesicles (DCV) and by their more extensive rough endoplasmic reticulum (RER). Cell bodies of both types formed axonal processes away from the NPO, but only type 2 neurons appeared to have ciliated cerebrospinal fluid (CSF)-contacting dendrites which terminated in the ventricle. The nucleus lateralis tuberis (NLT) could be divided into, six regions and comprised several ultrastructurally distinct cell types. Neurons containing 75-85 nm DCV were observed in the pars posterioris (NLTp), and large cell bodies with extensive arrays of distended RER and numerous DCV averaging 150 nm formed the pars lateralis (NLTl). The pars rostralis (NLTr) contained scattered cell bodies similar to, but smaller than, those of the NLTl. CSF-contacting neurons were also observed in the NLT. Both NPO and NLT were innervated by type B (aminergic) fibres. The nucleus recessus posterioris (NRP) and nucleus resessus lateralis (NRL) were composed of a single type of neuron, which contained numerous 80-nm DCV and displayed both long axonal processes towards the NLT and ciliated apical processes into the ventricle. The Falck-Hillarp method demonstrated that of the nuclei examined only the NRP and NRL showed strong monoamine fluorescence. Several fluorescent tracts were observed to run between the NRP and NRL and also towards the NLT and pituitary. It is proposed that these nuclei are the principal source of the hypothalamic and pituitary "type B" nerve endings, which are seen as fluorescent varicosities with the Falck-Hillarp method. Evidence for NPO and NLT cell bodies being the origin of the hypophysial peptidergic "type A" fibres is discussed, together with indications that these nuclei might be involved in control of adenohypophysial activity.