Experiments, involving para-chloro-D,L-phenylalanine-induced pup-killing (filicide), were conducted to determine the effect of preexposure to the goal entity on the initial development and long-term expression of PCPA-induced filicide. Three groups of Sprague-Dawley rats (n = 50) were either preexposed/not preexposed, or preexposed for varying periods (5, 8, or 13 days) prior to receiving PCPA. Two major results were obtained: 1) preexposed animals were less filicidal than those not preexposed; and 2) there was an inverse relationship between the number of preexposures and filicide occurrence. The results are interpreted in reference to a serotonin-depletion habituation hypothesis.