The effects of 5 s–24 h exposure to CuO nanosuspension (nCuO) and CuSO₄ on mortality of Nitellopsis obtusa cells within 48 days were investigated. Based on LED50 kinetics, a toxicological endpoint of lethal exposure duration that induces 50% cell mortality, it was shown that mortality response of 10-min treated cells (by 3.2 and 24–79.9 mg Cu/l, respectively CuSO₄ and nCuO) did not differ after 12th day rewash in control medium. Larger amount of total Cu (in mg/g cell DM) was measured in charophyte cells exposed to nCuO (79.9 mg Cu/l) than to ionic Cu (3.2 mg Cu/l). The lower nCuO-induced mortality than that of Cu²⁺ during earlier rewash period could be related to the sorption of nanoparticles on the cell wall, which was confirmed by scanning electron microscope images and energy dispersive X-ray spectrum data. Effect threshold, the shortest exposure duration that induces cell mortality significantly different from the control during a post-exposure period, comprised 30 s in 24 mg Cu/l of nCuO, however, it could not be identified and was less than 5 s in 79.9 mg Cu/l of nCuO.