The influence of thyroid status on the clearance of chylomicron 14C-labeled triglycerides (14C-TG), the fractional uptake of 14C-TG, and activity of lipoprotein lipase (LPL) in the different skeletal muscle fiber types was evaluated in pentobarbital-anesthetized rats. The turnover of plasma TG was approximately fourfold greater in the hyperthyroid (HyperT) compared to either euthyroid (EUT) or hypothyroid (HypoT) animals. The uptake of 14C-TG was increased in slow-twitch red and fast-twitch red muscle sections of the HyperT group and normal in the HypoT group. The changes in LPL activity in these two fiber types were inversely related to thyroid influence, with decreases found in the HyperT group and increases found in the HypoT group. Thus, the uptake of 14C-TG in these high-oxidative fibers, relative to the LPL activity, varied directly with thyroid influence. As a result, the normal linear relationship, apparent for the different types of skeletal muscle, between LPL activity measured in vitro and TG uptake determined in situ was not maintained.