A study was undertaken to learn whether the young Mg-deficient mammal can respond to major stress with increased levels of plasma corticosterone. Plasma corticosterone was determined in 48 weanling rats with dietary Mg deficiency and in 48 Mg-sufficient controls fed a Mg-supplemented diet, studying 12 animals at a time on experimental day 14. Each animal was studied once, either in an unstressed state or after the stress of audiogenic or strychnine seizures. Plasma corticosterone was determined using a radioimmunoassay; Mg was analyzed by atomic absorption spectrophotometry. On experimental day 14, unstressed Mg-deficient rats were tremulous, hyperirritable and showed slightly increased plasma corticosterone levels that exceeded controls levels. After strychnine shock, the mean plasma corticosterone levels of Mg-sufficient and Mg-deficient rats were both significantly increased over resting levels, and were not statistically different. Moreover, the spontaneous mortality rate that occurred during the experimental period in all Mg-sufficient rats was 0 compared to 27% among all Mg-deficient animals (p less than 0.0001). It was concluded that young rats deficient in Mg for 2 weeks could respond to major stress with levels of plasma corticosterone that were not significantly different from values of equally stressed Mg-sufficient controls. The deficient animals suffered a higher mortality, providing support for the concept that Mg deficiency increases stress-induced mortality in animals.