Plasma post heparin lipoprotein lipase activity (LLA) has been studied in patients with hyper- and hypothyroidism and in rabbits made thyrotoxic with thyroxine. The hypothyroid patients had high triglyceride and low LLA values as compared with a control group of healthy subjects. Statistically highly significant negative correlation was found between the triglycerides in fasting patients and the post heparin LLA, indicating a causal relationship, possibly with disturbance of chylomicron degradation due to low LLA in the arterial wall. However, relatively low LLA values were also demonstrated in hyperthyroid patients as well as in rabbits following treatment with thyroxine for 3 weeks. A stimulating effect of the thyroid hormones on the synthesis and degradation of lipoprotein lipase may be a possible explanation for these apparently contradictory findings.