Formation constants of copper(ii), zinc(ii) and calcium(ii) with 3,5-diaminodiamido-4-oxahexacyclododecane (cageL) has been studied by glass electrode potentiometry at 25 degrees C and an ionic strength of 0.15 mol dm(-3). Copper(ii) forms more stable complexes with cageL than zinc(ii) and calcium(ii). Metal ion complexation promotes deprotonation and coordination of the amide nitrogens resulting in overall tetragonal coordination of Cu(2+) suggested by the UV-visible electronic spectra. Speciation calculations using a blood plasma model suggest that zinc(ii) and calcium(ii) are good competitors of copper(ii) in vivo. Bio-distribution experiments using (64)Cu-labelled Cu(ii)-cageL show that about 50% dose of the complex is retained in the body after 24 h.