The medicinal leech (Hirudo medicinalis) is sensitive to various environmental changes. However, investigations on these leeches are restricted because of their scarcity in natural waters. Artificial of leeches makes possible the use of these animals in laboratory research. The aim of the present study to investigate changes of feeding behaviour and excretion rate following feeding in medicinal leeches exposed to different samples of water from Lake Drukshiai which provides the cooling waters for the Ignalina Power Plant (NPP). The following behavioural and physiological responses of medicinal leech exposed to of the Ignalina Nuclear Power Plant sampled in May and July 1997 were recorded: 1) feeding behaviour, that prolongation of the attachment process or a completely suppressed attachment reflex; 2) the size of blood meal; excretion rate after feeding. The most harmful changes of indices under study were caused by water from rain sewage: the highest percent of individuals with prolonged attachment process or completely attachment reflex; the smallest size of blood meal; the slowest excretion rate after feeding. Less severe were recorded after exposure to water from a waste water treatment plant. Water from Station 1, the remote monitoring station from Ignalina Nuclear Power Plant, showed the lowest effect on the medicinal in the majority of trials. The medicinal leech, due to its sensitivity, simplicity of measured indices and ease of maintenance can be used as a test-organism in studies of toxicity of polluted water.