Sperm-positive rats were intubated with Mirex in oil (5 to 10 mg/kg) on days 8-1/2 to 15-1/2. Controls were untreated or oil-fed. Testing was done on day 18-1/2. Fetuses were sequentially exposed and ECG's obtained with the fetus attached to the placenta and uterus. Counterparts of standard leads, I, II and II were used. Fetuses were weighed and examined afterwards. Swollen fetuses were rated on a scale of 1 (slight edema under chin) and 5 (3 mm edema across back). ECG's from 81 controls and 205 Mirex fetuses were obtained. They were evaluated for rate of heart beat, regularity of beat, PR intervals and other features. One control exhibited an abnormality, a transitory period of premature atrial contractions. Mirex-fed fetuses exhibited tachycardia, closely correlated with degree of edema. The heart rate increased from 150/min in controls to 180 in slightly swollen to 224 in swollen fetuses. Mean PR intervals increased with degree of swelling and with dose. The frequency of first degree heart block was also dose-related, ranging from 20% to 77%. Second degree heart blocks were found in 8%, 3% showed arrhythmias, and one had atrial flutter/fibrillation. These cardiovascular problems seem primarily related to Mirex-induced edema and demonstrate that fetal edema is not innocuous. These data demonstrate the usefulness of fetal electrocardiography to detect functional teratology.