Due to the increased use of ultrasound in the diagnosis of cyesis, it is necessary to study the possible adverse effect of insonation on thyroid hormone in fetuses. Sixteen (16) adult rabbit does that had kindled at least once were used for the study and were scanned using a Medison S600V® Ultrasound machine with a 6.5MHz transcutaneous curve-linear probe. All does were mated naturally by the introduction of a doe to a buck and then divided randomly into two groups. The does that were scanned (insonated)-, were properly restrained on dorsal recumbency, liberally shaved; aquasonic gel applied and scanned using a transcutaneous curve-linear probe for 5 minutes on the 5th day post coitus and thereafter on days 7, 12, 15, 20, 25, 27 and 29. The restrain and scanning procedures were mimicked on the group of does that were not scanned (non-insonated). Upon parturition, blood was collected from kits and serum was extracted and used to assay for total thyroxine (tT4) and triiodothyronine (tT3). Fetal thyroid glands were also harvested for histologic study. Mean tT3 concentration was 1.60 ± 0.90 µg/dl and 1.58 ± 0.03 µg/dl while mean tT4 was 1.46 ± 0.10 µg/dl and 1.39 ± 0.07 µg/dl for the non-insonated and insonated groups, respectively. P values of 0.610 and 0.132 for tT3 and tT4 concentration respectively showed that there is no statistically significant difference in the values even though there was a numerical difference between the groups. No histopathologic change on fetal thyroid gland from both groups was observed. In conclusion, although the values were not significantly different, the numerical decrease of both tT3 and tT4 in the insonated rabbits suggest the possibility that insonation could alter their values in fetuses when the threshold of ultrasound waves are increased and insonation prolonged.