An apparent increase in the ultrasound attenuation coefficient per unit frequency, alpha f, of fetal liver as a function of gestational age has been observed. Measurements were made in utero with a 25 megasample/sec RF digitizer and a real time ultrasound system with a 5 MHz scan head. A precise measurement of alpha f was employed in which the intercept was tied to 0 at a frequency of 0. In 178 examinations of normal pregnancies, the linear regression of the alpha f increased 26% between 26 and 40 weeks gestation. This statistically significant increase (p less than 0.0001) is consistent with several observations, those of Parker et al. of increased attenuation in liver when glycogen is added, the increasing glycogen storage in the liver before birth, and our own pre- and postnatal measurements reported elsewhere. A noninvasive assay for glycogen content would have important applications in medicine and biomedical science. However, an increase in measurement accuracy and precise correlation with glycogen content will be required to make meaningful predictions in individual cases, as opposed to the present statistical trends.