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Red cell G6PD enzyme activity was studied for 20 weeks in 12 5-week old minature swine littermates receiving isocaloric, cyanide (CN-) variable rations at proportions of dietary CN - approximating the cassava- derived ingestion levels of Liberian human groups. The hematological status and G6PD isozyme patterns were identical for all animals. Serum thiocyanate levels were positively correlated with dietary CN- intake loads (r=.83, p<0.01). Over the course of the experiment, high treatment animals receiving 1.2 mg CN - /day/kg body weight demonstrated significantly depressed overall mean enzyme activity (p<0.025) when compared to con­trol animals (0 mg cyanide), particularly during the first 12 weeks of the study. After week 16, the rate of enzyme inactivation in high treatment animals declined and activity levels began to converge with those of control animals. For animals receiving 0.4 to 0.7 mg CN-/day/kg body weight, red cell G6PD enzyme inhibition was initially delayed, however by week 20 activity levels were lower than either the control or high treatment groups. A less pronounced general decline in enzyme activity was observed in all animals and associated with aging. The possible health and evolutionary implications of these enzymatic responses to sublethal dietary CN- are discussed.