Cadmium (Cd), triethyltin (TET), and trimethyltin (TMT) are heavy metals which are neurotoxic to developing animals. In the present experiment, preweaning assessment of locomotor activity was used to detect and differentiate between the developmental toxicity of these metals. On postnatal day (PND) 5, rat pups received a single injection of either Cd, TET, or TMT. A within-litter design was used for dosing; 1 male and 1 female pup from each litter (N = 10 litters/compound) received either the vehicle, low, medium, or high dosage of the compound. Preweaning motor activity was assessed in 30-min sessions in figure-eight mazes from PND 13 to 21. Motor activity of control animals progressively increased in the initial days of testing, and then both within-session and between-session habituation developed. A single exposure to Cd, TET, and TMT produced hyperactivity by the end of the preweaning period but these metals differed in the day of peak activity, the onset of hyperactivity, and the development of habituation.