Mammillary body neurons projecting to the thalamus were identified by injecting retrograde tracers into the medial thalamus of macaque monkeys. The source of the thalamic projections from the medial mammillary nucleus showed strikingly different patterns of organization depending on the site of the injection within the two anterior thalamic nuclei, anterior medialis and anterior ventralis. These data reveal at least two distinct modes by which the primate medial mammillary bodies can regulate anterior thalamic function. Projections to the thalamic nucleus anterior medialis arise mainly from the pars lateralis of the medial mammillary nucleus. A particularly dense source is the dorsal cap in the posterior half of the pars lateralis, a subregion that has not previously been distinguished. In contrast, neurons spread evenly across the medial mammillary nucleus gave rise to projections more laterally in the anterior thalamic nuclei. A third pattern of medial mammillary neurons appeared to provide the source of projections to the rostral midline thalamic nuclei. In contrast, the labeled cells in the lateral mammillary nucleus were evenly spread across that nucleus, irrespective of injection site. In addition to the established projection to anterior dorsalis, the lateral mammillary nucleus appears to project lightly to a number of other thalamic nuclei, including lateralis dorsalis, anterior medialis, anterior ventralis, and the rostral midline nuclei, e.g. nucleus reuniens. These anatomical findings not only reveal novel ways of grouping the neurons within the medial mammillary nucleus, but also indicate that the mammillothalamic connections support cognition in multiple ways.