Weights of kidneys, heart, lungs, liver, adrenals, eyes and tongue of the northern form of franciscana(Pontoporia blainvillei) and the estuarine dolphin (Sotalia guianensis) incidentally caught in fishing nets arepresented and related to total body weight. Because of the smaller body size of the northern form ofP. blainvillei, its actual organ weights were all smaller than those of the southern form reported in theliterature. However, the lungs and adrenals of the northern form were proportionally greater than thoseof the southern form. The relative weight of the males' left kidney (N=18) and females' liver (N=17) weresignificantly heavier in this study than the proportions presented elsewhere in the literature for thesouthern form of the species. However, the females' kidneys, males' right kidney and males' liver did notdiffer between the northern and southern forms, and a larger number of individuals of both geographicalforms should be analysed. The actual weights of the S. guianensis internal organs were all greater than thosereported in the literature for S. fluviatilis. Additionally, the body-to-organ mass proportions of kidneys,heart and lungs of S. guianensis were also greater than those of S. fluviatilis. This study provides comparativedata that can be used when examining the systematics of both species and provides a baseline for futureexaminations of the post-mortem conditions of these cetacean species.