The ontogeny of the tonic immobility (TI) response in domestic fowl chicks was studied during the first week of life. The TI response of naive White-Leghorn Gallus domesticus male chicks (N=5–9), was tested at the age of 1, 2, 3, 5 and 7 days. TI was induced dorsally and its duration, the number of induction trials and the latency of peeping were recorded. The TI response was strongly affected by age. It was poorly developed during the first 3 days of life, when the median TI duration in control chicks was 10 s and the mean number of induction trials 2.3±0.3. After the third day of life, TI duration increased by up to 15× and susceptibility by about two. Peeping latencies were very short throughout the first week and in many cases, peeping started long before the termination of TI. Immediately following recovery from TI, chicks were put in an open field and the latencies of walking and jumping and the number of steps, jumps and peeps were observed. No changes in either locomotion or vocalization in an open field were found between the third and fifth day. Furthermore, there was no correlation between any of the parameters of the TI and OF tests. The effect of habituation, which is known to attenuate the TI response, was studied by repeatedly subjecting chicks to TI and OF tests, once on each day of the experiment. Habituation prevented the increase in TI duration and susceptibility after the third day of life, but did not affect the OF response. The effect of aversive treatment, which was expected to increase TI, was examined by placing chicks in 5-cm deep tap water for 5 min, prior to testing. Treatment significantly attenuated TI on Day 1 and increased overall locomotion and peeping in open field.