Orphaned or abandoned harbor seal pups (Phoca vitulina richardsi) rescued by the California Marine Mammal Center (CMMC) were studied during a four week period postnatally. Under normal conditions in the wild this period for the pups is one of rapid growth, laying down of blubber and molting of lanugo coat. The animals examined at CMMC in this study suffered from a respiratory distress syndrome, the etiology of which is as yet unknown. Neonatal death was a common occurrence. The animals failed to grow rapidly, deposited little blubber and molted lanugo very slowly. By investigating serum thyroxine (T4) and triiodothyronine (T3) levels in both survivor and non-survivor pups, we will attempt to shed light on whether the thyroid plays a part in this respiratory distress syndrome.