Fig. 1.

Two experimental designs used to study each of 13 chemicals are outlined above. The upper panel describes the “3 × 1” design. These studies entailed exposing rats to test agent for three consecutive days, obtaining blood samples one day later, and obtaining blood and liver tissue 4 days after cessation of treatment. The lower panel illustrates the “3 × 3” design.” In this case, three days of exposure occurred in each of three consecutive weeks. As with the 3 × 1 studies, blood was collected on Day 4, and blood and liver were also collected 4 days after cessation of treatment on Day 21 of the study.