Serial measurements of haematocrit (Ht), plasma thyroxine (T4), triiodothyronine (T3) and alpha-hydroxybutyrate dehydroxygenase (alpha-HBDH) were performed in patients following myocardial infarction (MI). Infarct size was estimated by mathematical analysis of the change in plasma alpha-HBDH activity with time. After an initial small increase Ht decreased 12% until day 9 and remained constant thereafter. Serum T4 did not change during the entire study. Serum T3 decreased to 66% at day 9 and then returned to normal within 2 months. These figures are expressed relative to determinations in the first blood sample obtained within 12 h after MI. A significant correlation between the lowest serum T3/T4 ratio and infarct size was observed. These observations suggest that in these patients the peripheral conversion of T4 into T3 is reduced. This was accompanied by an increased production of reverse T3 as evidenced by observations in one patient.