The thymidine analog 5-bromo-2'-deoxyuridine (BrdUrd) is felt to exert its cytotoxic effects primarily through incorporation into DNA. We have evaluated the incorporation of BrdUrd into the DNA of relevant normal tissues (bone marrow, gut mucosa, and liver) and tumor in rabbits with the VX2 tumor growing intrahepatically. Using constant i.v. infusions, steady state plasma drug concentrations ranging from 0.4 to 65.4 microM were maintained for 24 h and tissues were harvested and processed so that a sensitive gas chromatography/mass spectrometry (GC/MS) method could be used to analyze the thymine and 5-bromouracil content of hydrolyzed DNA. In all tissues, DNA incorporation showed saturating effects as plasma BrdUrd concentration was increased and, BrdUrd incorporation as a function of plasma concentration could be fitted to a Langmuir-like equation generating tissue-specific pharmacodynamic parameters: Imax for percentage thymine replacement at infinite plasma BrdUrd concentrations, and C50 for the arterial BrdUrd concentration generating incorporation that is Imax/2. At all plasma concentrations of BrdUrd the incorporation into DNA of bone marrow was greater than that observed in VX2 tumor. However, BrdUrd labeling index (with a BrdUrd monoclonal antibody) was greater in tumor than bone marrow. Thus, pharmacodynamic differences in incorporation do not result solely from cytokinetic differences between tissues. This model may prove useful in evaluating the pharmacodynamics of incorporation in studies using hepatic arterial infusion and biochemical modulation to improve selectivity.