In 21 males (age 50-78 yrs) without endocrine diseases, plasma thyroxine, triiodothyronine, reverse-triiodothyronine and TSH were determined by specific RIAs in the early stage of an acute myocardial infarction. Blood was taken every 4 hours up to 43 hours after admission. The patients were separated in 2 groups, depending on the severity of the infarction (group A with 11 patients = severe infarction; group B with 10 patients = not severe infarction). 25 healthy men served as an aged-matched control group (age 56-83 yrs). Thyroxine in all patients (group A and B together) did not show essential fluctuations during the 43 hours of observation. The mean values were elevated, the individual values were predominantly in the normal range. There were no significant differences between the groups A and B. As well in the total-patients group as in the groups A and B already on admission, T3 was diminished and went on falling during the 43 hours. From 35 hours after admission, T3 was significantly decreased in group A compared to group B. On admission, rT3 was elevated in both the total-patients group and group A and B; it reached its maximum after 19 hours. Group A showed from 7 hours after admission a tendency more increased levels compared to group B. In all cases (total-patients group, group A and B) TSH levels were constant within the normal range during the whole time. The results suggest that in the early stage of myocardial infarction an important alteration in the thyroxine metabolism with enhancement of the inactive reverse-triiodothyronine production occurs very quickly. Moreover, the results show that this alteration may depend on the severity of myocardial infarction. Hypothyreoidism can be excluded because of the normal or elevated thyroxine values and the normal TSH levels.