Acute, subacute, in vitro, and animal experiments were performed in the spring and fall of 1981. In the course of these investigations, samples from bank-filtered drinking-water wells of Vác, a town along the Danube; from three neighboring observation wells; from water transported from the opposite side of the Danube; from a drinking-water well of a town farther northward; and from Danube water were examined. The aim of the study was to detect whether any of the water samples became contaminated by toxic chemicals. The most severe changes were found in two observation wells (G4 and G2). The drinking-water samples of the wells in Vác (Nos. 1-5) proved to be contaminated, and toxic effects of different degrees were also induced by other wells of Vác. The Danube water caused changes in some tests only; the control drinking-water well, the samples of the K3 observation well, and of the well on the opposite side of the Danube examined in the fall only did not cause any alterations. The toxicity was confirmed by the elevated hemolysis by the genotoxic effects on human erythrocytes and leukocytes, by the functional changes observed in the nervous system of rats, by alterations detected in the electric activity of the rats' brains, in the learning ability and the behavior of the rats, by the increase in the cytochrome P-450 enzyme level of the liver, and by the chromosome aberrations emerging in mice. The changes were caused by different compounds of low and high boiling points escaping with water vapor or with organic solvents. On the basis of the investigations, the water proved to be inappropriate for human consumption.