Nursing rat mothers were injected with penfluridol, a dopamine receptor blocking agent, at Day 1, 3, 5 and 7 after delivery, and their offspring were investigated at 6-18 days of age for various aspects of motor behavior development, and at 4, 8 and 12 weeks of age for open field behavior. No deficits were found in the development of locomotion, air righting, startle response and eye opening. Open field ambulation decreased from an abnormally high level at 4 weeks of age to an abnormally low level at 8 and 12 weeks of age. The ability to habituate to an open field decreased from a normal level at 4 weeks of age to an abnormally low level after 8 weeks of age. The behavior deficits were related to a decreased functional activity of the mesolimbic dopamine neuron system, and the possible relation to a clinical dysfunction among children - minimal brain dysfunction - was discussed.