New Findings: What is the central question of this study? Does passive heat stress of +2°C oesophageal temperature change concentrations of circulating arterial endothelial- and platelet-derived microparticles in healthy adults? What is the main finding and its importance? Concentrations of circulating endothelial- and platelet-derived microparticles were markedly decreased in heat stress. Reductions in circulating microparticles might indicate favourable vascular changes associated with non-pathological hyperthermia. Interest in circulating endothelial- and platelet-derived microparticles (EMPs and PMPs, respectively) has increased because of their potential pathogenic role in vascular disease and as biomarkers for vascular health. Hyperthermia is commonly associated with a pro-inflammatory stress but might also provide vascular protection when the temperature elevation is non-pathological. Circulating microparticles might contribute to the cellular adjustments and resultant vascular impacts of hyperthermia. Here, we determined whether circulating concentrations of arterial EMPs and PMPs are altered by passive heat stress (+2°C oesophageal temperature). Ten healthy young men (age 23 ± 3 years) completed the study. Hyperthermia was achieved by circulating ∼49°C water through a water-perfused suit that covered the entire body except the hands, feet and head. Arterial (radial) blood samples were obtained immediately before heating (normothermia) and in hyperthermia. The mean ± SD oesophageal temperature in normothermia was 37.2 ± 0.1°C and in hyperthermia 39.1 ± 0.1°C. Concentrations of circulating EMPs and PMPs were markedly decreased in hyperthermia. Activation-derived EMPs were reduced by ∼30% (mean ± SD; from 61 ± 8 to 43 ± 7 microparticles μl−1; P < 0.05) and apoptosis-derived EMPs by ∼45% (from 46 ± 7 to 23 ± 3 microparticles μl−1; P < 0.05). Likewise, circulating PMPs were reduced by ∼75% in response to hyperthermia (from 256 ± 43 to 62 ± 14 microparticles μl−1). These beneficial reductions in circulating EMPs and PMPs in response to a 2°C increase in core temperature might partly underlie the reported vascular improvements following therapeutic bouts of physiological hyperthermia.