<p>Tl⁺ is thought to be toxic to cells due to ionic mimicry of K⁺. The aims of this study were two-fold. First, to identify whether K⁺ and Tl⁺ were interacting in isolated guts, whole animals and tissues in <em>Chironomus riparius, </em>and second, to determine the strategies of Tl⁺ tolerance. <em>C. riparius. </em>were very tolerant towards Tl⁺with a 48-hr LC₅₀ of 723 μmol l^-1. The Scanning Ion-selective Technique (SIET) allowed us to identify the caecae, AMG and PMG as the major K⁺-transporting regions of isolated guts. Evidence for an interaction was based on the finding that Tl⁺ was transported in the same directions at these segments (and others), and that 50 μmol l^-1Tl⁺ decreased K⁺ flux at the AMG and PMG. In addition, exposure to Tl⁺ prior to flux measurements had significant effects on net K⁺ transport by the gut. Measurements of Tl⁺ and K⁺ concentrations in the whole animal, gut and hemolymph by Atomic Absorption Spectroscopy (AAS) indicated that Tl⁺ uptake was saturable in the whole animal and gut, and non-saturable in the hemolymph. Together with the SIET measurements, the AAS data suggests that high levels of Tl⁺ can perturb K⁺ transport and homeostasis. The absorption of Tl⁺ from the gut to hemolymph, measured by SIET, was confirmed by hemolymph measurements of Tl⁺ using AAS. This indicated that Tl⁺ gains access to the hemolymph and that sensitive tissues (such as the nervous system) are thus exposed. However, survival of <em>C. riparius</em> at these concentrations implies efficient mechanisms for detoxification of Tl⁺. This tolerance may involve sequestration in the gut, metal-binding proteins and increased secretion by the anal papillae and MTs. In addition, loss of K⁺ from the muscle may prevent hypokalemia in the hemolymph and gut.</p> / Master of Science (MSc)