An experiment was conducted to test methionine source and concentration on the growth performance and physiological responses of heat-stressed broiler chickens. Birds were fed either DL methionine or 2-hydroxy-4-methylthiobutanoic acid (HMB) at two concentrations (adequate: starter 0.94%, grower 0.84% or superadequate: adequate concentrations +20% methionine) over a six day period at normal or elevated environmental temperatures. Heat treatment was initiated at 21 days of age within controlled environmental chambers. Blood samples were taken 6 hours and 6 days post treatment to determine the effects of acute and chronic heat stress on various blood parameters. Heat treatment significantly impaired growth performance over the 6 day period. There were no effects of methionine source or concentration on the growth performance of broilers within any of the environmental treatments. Blood parameters changed during acute heat stress, but normalized over chronic heat exposure. Birds were apparently able to adapt blood chemistry to heat exposure subsequent to chronic heat stress, but were still negatively affected from a performance standpoint.