This work investigated the plasma oxolinic acid (OXA) concentrations achieved by multiple administrations of medicated feed to laboratory held rainbow trout (Oncorhynchus mykiss) at 6 degreesC. These data were compared with previously published data obtained with trout held at 12 degreesC. Both sets of fish were administered oxolinic acid at 50 mg/kg at 1% body weight on six occasions during a 9-day period and sampled 1 day after the last administration of medicated feed. The mean plasma OXA concentrations were 0.14 +/- 0.07 and 0.15 +/- 0.10 mg/l while the 80th percentiles were 0.09 and 0.08 mg/l, at 12 degreesC and 6 degreesC, respectively. The extent of fish to fish variation, measured as the percentage coefficient of variation (CV), were 47% and 66% at the two temperatures. These data indicate that temperature, over the range 6-12 degreesC is not a significant factor in determining plasma concentrations achieved following multiple oral administrations of medicated feed. It is argued that a single breakpoint MIC value could be applied to any therapeutic treatment performed within this temperature range. (C) 2004 Elsevier B.V. All rights reserved.