Chronically catheterized rainbow trout (Oncorhynchus mykiss) were exposed to 4 mg/l (nominal concentration) of Zn2+ and their arterial oxygen tension measured. When the arterial oxygen tension had decreased to less than 50% of the original, one group of fish was sacrificed, and the structure of the gill secondary lamellae analyzed morphometrically. Two other groups of animals were allowed to recover in clean water and their arterial oxygen tension measured. After a recovery period of 24 h or 48 h the fish were sacrificed and their gills analyzed morphometrically. Associated with the decrease in arterial oxygen tension in the exposure, there was a marked increase in the arithmetic mean distance from water to blood across the gill epithelium, from 4.2 mu m in control animals to 9.8 mu m in exposed animals, and a consecutive decrease in the calculated morphometric diffusion capacity of the secondary lamellae. In clean water, the arterial oxygen tension fully recovered within 48 h, whereas the arithmetic mean distance from water to blood, although it decreased significantly as compared to the value at the end of the exposure, remained at an elevated level even after 48 h recovery (5.6 mu m) as compared to control animals. Thus, in resting fish, diffusion limitation starts to influence the arterial oxygen tension when the arithmetic mean distance from water to blood is approximately 6 mu m.