The current risk assessment of compounds is generally based on external exposure and effect relationships. External doses are often not representative for internal exposure concentrations. The aim of this study was to show how the implementation of toxicokinetics in a scheduled toxicity study contributes to improved data interpretation without additional use of animals and to the three goals of the 3R principles for animal testing. Toxicokinetic analyses were implemented in a rat developmental immunotoxicity study with 4-methylanisole without interfering with the outcome of the study and without the use of additional animals. 4-Methylanisole and its metabolites were analysed in plasma of adult rats and in pups at postnatal day 10. 4-Methylanisole has a short half-life in adult animals and the plasma concentrations increased more than proportional with increasing dose. The metabolic profile appeared to be different at low dose as compared to high dose. This information on the dose-proportionality of the internal exposure is crucial for the interpretation of the toxicity data and helps to identify the toxic agent and the appropriate dose metric. The metabolism was similar in adult and juvenile animals. Large inter-individual variability in adult animals, as observed for 4-methylanisole, may hamper dose–response analyses of the results. In addition, 4-metylanisole was excreted via milk, but concentrations in the juvenile animals appeared to be 20- to 100-fold lower than via direct gavage exposure. The toxicokinetic parameters support the data interpretation, among others by providing better insight into internal exposures. Subsequently, it will help to prevent testing of irrelevant exposure scenarios and exposure concentrations. Overall, implementation of kinetics with limited effort provides useful information to support the interpretation of toxicological data and can contribute to reduction and refinement of animal testing.