<p>Potassium cyanide (KCN) is a highly lethal poison with cyanide anions having an inhibitory effect on complex IV of the mitochondrial respiratory chain, leading to stoppage in electron transport and eventually cessation of aerobic respiration within the cell. Tardigrades are a group of small invertebrates, most well known for their exceptional resistance to environmental stressors, including exposure to aqueous solution of KCN. In this study, specimens of the tardigrade <i>Paramacrobiotus experimentalis</i> were subjected to KCN exposures of various concentrations and durations, as well as repeated exposures. The resulting reactions have been observed, both by observing its movements and through ultrastructure analysis using transmission electron microscope (TEM). Obtained results confirm high tolerance of tardigrades to KCN. After an initial period of debilitation, tardigrades gradually return to full activity. Statistically significant relationships between time needed for recovery and KCN concentration, duration of exposure and number of consecutive exposure episodes have been found. However, no significant relationship between KCN exposure and long-term survival has been found. Analysis using TEM has found changes in midgut and storage cells of exposed animals, including mitochondrial damage and evidence of autophagy. Finally, a new protocol for tardigrade exposure to KCN has been devised.</p>