Sodium fluoride (NaF) and sodium dichromate (Na2Cr2O7) are two different toxic compounds which are used as a dental caries prophylactic and as an oxidising agent in various industrial areas, respectively. However, accidental fluoride and chromate poisoning is not a rare occurrence, even death may result from cardiac or respiratory failure. In the present work, alterations produced by NaF, Na2Cr2O7 and temperature changes in the molecular dynamics of the human erythrocyte membrane were studied, in vitro, by the spin-labelling ESR technique. Human intact erythrocyte cells spin labelled with 5- and 16-doxyl stearic acids (5-DSA and 16-DSA) and treated with 40 mu M NaF and 5 mu M Na2Cr2O7 at 37 degrees C were used to quantify membrane fluidity. This was performed by measuring the changes in the order parameter (S), correlation time (tau) and phase transition temperature using recorded electron spin resonance (ESR) spectra. Experimental results show that 5 mu M Na2Cr2O7 and 40 mu M NaF do not produce any significant effects on the order parameter of 5-DSA spin label while they cause appreciable changes in the correlation time of the same label. As for 16-DSA, while Na2Cr2O7 does not produce any measurable effect on the order parameter of this label, NaF does in a certain extent. Although weak, the effects of both compounds on the correlation time of 16-DSA are found to be well above the experimental error limits. Change in temperature was observed to alter significantly S and tau parameters which show biphasic character in the temperature range of 5-50 degrees C. Activation energies of the hydrocarbon chains above and below transition temperatures were also determined for untreated and NaF or Na2Cr2O7 treated erythrocyte cells and the effect of NaF and Na2Cr2O7 on these energies and transition temperatures were discussed. (C) 2000 Elsevier Science B.V. All rights reserved.