Abstract o 1. Concentrations of plasma lipid and plasma protein, and plasma protein composition were studied in two populations of Belding ground squirrels at high altitude throughout their season of activity. 2. Plasma lipid was highest in lactating females and tended to vary otherwise in direct relationship to total body lipid. 3. Plasma protein decreased during the first weeks after emergence from hibernation and increased thereafter. It is suggested that this was related to regulation of circulating crythrocytes. 4. There was some seasonal variation in all protein fractions, with greatest lability seen in α2-globulin followed by albumin. Squirrels living at 2130 m exhibited a prealbumin fraction more frequently than those from 3020 m. 5. Seasonal cycles in all plasma constituents tended to be more pronounced in squirrels living at the higher altitude.