Liver samples were obtained from Sprague-Dawley weanling rats of both genders that were fed individually various concentrations of PCB 28, 77, 105, 118, 126, 128 or 153 daily for 13 weeks, and processed by conventional techniques for transmission electron microscopy.  Hepatocyte alterations in females comprised    elevated   mean   volume   fraction  (VF) of smooth endoplasmic reticulum (PCB 28, 105, 118, 126, 128, and 153), rough endoplasmic reticulum (PCB 128), mitochondria (PCB 77 and 126), peroxisomes (PCB 118 and 153), and lipid droplets (PCB 105 and118).  Dietary exposure to PCB 153 at 50 ppm in females induced heightened VF of abnormal liver mitochondria that contained aberrant cristae.  Also, a significant decrease in VF of normal mitochondria was induced by congener105 at 50 ppm in the females.  Congener 128 significantly increased VF of rough reticulum profiles in males.  In general,  PCBs altered  the liver morphology of  female rats at concentrations that did not affect the morphology in corresponding males.  Using hepatocyte morphology VF as an index, we propose the following ranking of congener potency for Sprague-Dawley female rats: PCB126 >118 > 77>28 <105 <128 <153.  Information obtained from our studies may be used to develop a predictive model for assessing the effects of these congeners on human beings, and by extrapolation of these data, may be used to  determine   the  no  observable  adverse  effect  level (NOAEL) in humans.