The effects of two widely used paramagnetic shift reagents for cationic NMR, dysprosium tripolyphosphate [Dy(PPP)2(7-)] and dysprosium triethylenetetramine hexaacetate [Dy(TTHA)3-], on the cell structure of dog and human erythrocytes, dog kidney cortical tubules and rat hepatocytes were investigated. The effect of shift reagents on cell integrity was monitored by measuring the hematocrit values for erythrocytes, by measuring the lactate dehydrogenase (LDH) release and by electron microscopy for cortical tubules and hepatocytes. The quantitation of the dyprosium penetration inside cells was accomplished by atomic absorption, atomic emission and neutron activation. More severe effects were observed with Dy(PPP)2(7-) than with Dy(TTHA)3-, and were dependent on the divalent cation concentration and on the shift reagent concentration. Very serious cell damage was observed after 60 min incubation in the presence of 10 mumol Dy(PPP)2(7-)/mL suspension at low or high divalent cation concentration. The situation was improved at 5 mumol Dy(PPP)2(7-)/mL suspension especially at high divalent cation concentration (2.5 mM). Incubation with Dy3+, PPP5- or Dy(TTHA)3- caused little or no structural effects but dysprosium was found to penetrate slowly inside tubules with Dy(TTHA)3-. Both Dy3+ and Dy(PPP)2(7-) penetrated rapidly inside cells. Dysprosium was found to bind to the isolated cytosol but not to isolated membranes, eliminating the possibility of extracellular membrane binding.