The physiologic and clinicopathologic effects of weathered South Louisiana crude oil exposure were studied in the laboratory in juvenile loggerhead sea turtles. Sea turtles ingested oil incidentally, and oil was observed clinging to the nares, eyes, and upper esophagus, and was found in the feces. Oiled turtles had up to a four-fold increase in white blood cell counts, a 50% reduction in red blood cell counts, and red blood cell polychromasia. Most serum blood chemistries (e.g., BUN, protein) were within normal ranges, although glucose returned more slowly to baseline values than in the controls. Gross and histologic changes were present in the skin and mucosal surfaces of oiled turtles, including acute inflammatory cell infiltrates, dysplasia of epidermal epithelium, and a loss of cellular architectural organization of hte skin layers. The cellular changes in the epidermis are of particular concern because they may increase susceptibility to infection. Although many of the observed physiological insults resolved with a 21-day recovery period, the long-term biological effects of oil on sea turtles remain completely unknown.