The ontogeny of habituation in the neonatal rat, using the forelimb-withdrawal response, was examined. Thresholds and latencies of the response, changing patterns of responding to shock stimuli and habituation of the response were studied in rats 3-15 days of age. Although response thresholds do not change during this period of development, response latencies decrease and amplitudes increase. Three and 6 day old pups remain active much longer following shock than do 10 and 15 day olds. When compared for habituation to different frequencies of stimulation, 3 day old rat pups were much more susceptible to habituation at low frequencies than 15 day olds. Insertion of a simple intense shock in the habituation series produced marked dishabituation in the 3 day olds but little or none in the 15 day olds. This pattern of results was obtained regardless of the locus of the dishabituating shock. Analysis of response latencies showed that although 15 day olds responded 2-3 times more quickly than 3 day olds, both age groups responded more rapidly to a strong shock than to a weak one. During habituation, response amplitudes of both 3 and 15 day old pups declined with no change in latency. Age-related differences in habituation were independent of differences in reactivity to shock and more likely due to the emergence of response sensitization.