The ascending projections of cholinergic neurons in the laterodorsal tegmental nucleus (TLD) were investigated in the rat by using Phaseolus vulgaris leucoagglutinin (PHA-L) and wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP) anterograde tracing techniques. Two ascending pathways were identified after iontophoretic injections of PHA-L into the TLD. A long projection system courses through the dorsomedial tegmentum, caudal diencephalon, medial forebrain bundle, and diagonal band. Different branches of this system innervate the midbrain (superior colliculus, interstitial magnocellular nucleus of the posterior commissure, and anterior pretectal nucleus), the diencephalon (lateral habenular nucleus, parafascicular, anteroventral, anterodorsal, mediodorsal, and intralaminar thalamic nuclei), and the telencephalon (lateral septum and medial prefrontal cortex). The second system is shorter and more diffuse and innervates the median raphe, interpeduncular, and lateral mammillary nuclei. Retrograde tracing with WGA-HRP, combined with choline acetyltransferase immunohistochemistry, revealed that most of the TLD projections to the tectum, pretectum, thalamus, lateral septum, and medial prefrontal cortex are cholinergic. Afferents to the TLD were studied by anterograde and retrograde tracing techniques. Injection of tracers into the TLD retrogradely labelled neurons bilaterally in the midbrain reticular formation, the periaqueductal gray, the medial preoptic nucleus, the anterior hypothalamic nucleus, and the perifornical and lateral hypothalamic areas. Retrogradely labelled cells were also located bilaterally in the premammillary nucleus, paraventricular hypothalamic nucleus, zona incerta, and lateral habenular nucleus. In the telencephalon, the nucleus of the diagonal band and the medial prefrontal cortex contained retrogradely labelled neurons ipsilateral to the TLD injection site. The projections of the medial prefrontal cortex, the bed nucleus of the stria terminalis, and the lateral habenular nucleus to the TLD were confirmed in anterograde tracing studies. These findings indicate that the TLD gives rise to several ascending cholinergic projections that innervate diverse regions of the forebrain. Afferents to the TLD arise in hypothalamic and limbic forebrain regions, some of which appear to have reciprocal connections with the TLD. The latter include the lateral habenular nucleus and medial prefrontal cortex.