Manganese (Mn, atomic number 25) and rhenium (Re, atomic number 75) are group 7 (VIIB) transition elements. Before the discovery and confirmation of the existence of rhenium predicted by Mendeleev's periodic law, rhenium was provisionally termed dvi-manganese because of its expected resemblance to manganese. Manganese and rhenium share many of the general chemical characteristics of metals in the transition series, including multiple valency, the ability to form stable complex ions, paramagnetism, and catalytic properties. However, the second and third elements in the transition series generally have chemical properties more similar to each other than to the first member. Thus, in many respects, rhenium is chemically more similar to technetium than to manganese. Inhalation of particulate Mn constitutes the dominant route through which toxicity is expressed under most occupational conditions. Manganese is notably toxic to the central nervous system (CNS) and also has effects on the respiratory system and on reproductive function. Numerous clinical cases of frank Mn toxicity denote a characteristic syndrome that may include psychiatric symptoms, dystonia and rigidity, impaired manual dexterity, and gait disturbances. Several epidemiological studies provide a coherent pattern of evidence of neurotoxicity from occupational exposure to Mn at average concentrations around 1 mg/m3 or lower. The primary effects observed in such workers pertain to motor function, especially hand steadiness, eye–hand coordination, and rapid coordinated movements, which imply involvement of the CNS extrapyramidal system. Although a growing body of literature is devoted to medical applications of the radioactive isotopes 186Re and 188Re, very limited information is available on the toxicity of rhenium itself, which makes it difficult to characterize its toxicity with confidence. The few studies conducted thus far suggest that acute administrations of Re may have relatively low toxicity, at least by noninhalation routes. It has been described as “relatively inert” in the body and produces transient changes in blood pressure (both hypo- and hypertensive), tachycardia, sedation, and ataxia. In one comparative study, the lethal oral dose of Re was about eight times higher than that of molybdenum. However, one report suggests that it could be more potent as an inhalation toxicant. If true, rhenium and manganese might share the feature of having much greater toxicity by inhalation than by ingestion. Keywords: Manganese; Manganese compounds; Rhenium; Rhenium compounds; Nonhuman primates; Clinical cases; Edipemiology; Occupational exposure limits