Aldrin, a chlorinated hydrocarbon, inhibited rat brain synaptosomal membrane-bound Na+,K+ -adenosine triphosphatase (ATPase) and Mg2+-ATPase activities under in vivo and in vitro conditions. Na+,K+-ATPase was non-competitively inhibited whereas Mg2+-ATPase was inhibited uncompetitively. Arrhenius plots of both these ATPases without aldrin under in vivo and in vitro conditions were found to be linear. In the presence of aldrin, on the other hand, Arrhenius plots of the same ATPases were nonlinear. Slopes of Arrhenius plots of both ATPases under in vivo and in vitro condition were changed with change in temperature with aldrin. The activation energy (AE) of Na+,K+-ATPase and Mg2+-ATPase activities were changed over the temperature range 15-40° in the presence of aldrin. These results thus suggest that aldrin increases the lipid fluidity of the synaptosomal membrane which may be a cause of inhibition of neuronal membrane-bound Na+,K+ and Mg2+-ATPase activities. © 1990.