The effects of different thermal regimens on qualitative and quantitative characteristics of leucocytes were evaluated in two fish species of the Amazon region. 2. The proportion of circulating types of leucocytes changed significantly in Colossoma macropomum (tambaqui) but not in Hoplosternum littorale (tamoatá) exposed for four-week terms to 20, 25, 30, 35, and 40 degrees C. 3. The proportion of circulating lymphocytes decreased significantly in tambaquis exposed to 30, 35, and 40 degrees C. No changes in lymphocyte proportions were observed in tamoatás. 4. Neutrophils were almost absent in tambaquis, except in animals exposed to 40 degrees C. No significant changes in circulating neutrophils were observed in tamoatás. 5. The circulating leucocytes of Colossoma macropomum and Hoplosternum littorale are affected in different ways by temperature changes, suggesting species-specific adjustments to this parameter.
