We made a single intraperitoneal (IP) injection of morphine sulfate (40 mg/kg) into winter flounder and seawater acclimated rainbow trout at 10 degrees C and then followed its disposition by measuring the change in plasma morphine concentration for 100 h using a morphine specific ELISA. Disposition also was followed for 6h after a single IV injection of 7.5mg morphine sulfate in winter flounder. Plasma morphine reached a maximum within an hour post-injection IP and then decreased in a bi-exponential fashion with a rapid distribution phase followed by a slower elimination phase. The disposition was slower in flounder than in trout even though the fish were held at the same temperature. For example, plasma clearance was 76 mL h(-)(1) kg(-)(1) in the flounder but was almost twice as much in the trout (153 mL h(-)(1) kg(-)(1)) and mean residence time was 27.9h in the flounder but was 7.0 h in the trout. The present study is the first comprehensive pharmacokinetic analysis for any analgesic in an ectotherm, and our results show that: 1) significant intra-specific variation exists between fishes: and 2) the disposition of morphine in fish is approximately one order of magnitude slower than it is in mammals. These differences may be due in part to mass specific differences in cardiac output.