Seventy-eight Wistar weanling rats were pretreated with arsenate (100 mg/L As), selenite (1 mg/L Se), and arsenate (100 mg/L As) plus selenite (1 mg/L Se) added to the drinking water. After 4 w, all the animals were sacrificed and serum T3 and T4 were determined by double-antibody radioimmunoassay. Thyroid tissue concentrations of As and Se were determined in female rats by neutron activation analysis, and tissue specimens were examined histopathologically. For both sexes, the measurements indicated that T4/T3 was lowest in the Se group, intermediate in the As group, and highest in the controls. Corrected for the mean value of the controls, mean As concentration of thyroid tissue was of the same magnitude in the group pretreated with As + Se as the sum of the mean As concentration in the groups pretreated with As or Se alone. The outcome was symmetric with regard to the Se concentration: In the As + Se pretreated group, the mean Se concentration was of the same magnitude as the sum of the mean Se concentration in the groups pretreated with As or Se alone. Thus, As and Se tended to accumulate in the thyroid tissue. Postmortem examination showed that the thyroid tissue of rats pretreated with As alone exhibited obvious, toxic changes, whereas only minor or no changes were found in the tissues of the groups pretreated with Se or As + Se, and in the tissues of the controls. Multivariate analyses demonstrated that s-T4 and s-T3 were significantly correlated with sex, that s-T3 was positively correlated (p < or = 0.001) with Se pretreatment, and that the T4/T3 ratio was negatively correlated with both As (p < or = 0.012) and Se pretreatment (p < or = 0.001). The results were discussed in relation to the cancer preventive effect of Se.

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