AbstractNociceptive and antinociceptive systems change dramatically during the first 10 days of life in rats. The present studies sought to determine whether exposure to water stress can produce analgesia during this period, and when in the ontogeny changes in magnitude and duration of such stress‐induced analgesia occur. It was found that exposure to water reliably induced analgesia as early as 3 days postnatally. This analgesia increased in magnitude and duration at the age of 17–20 days, when supra‐spinal descending inhibition develops, and depended on the temperature of the water and duration of exposure. © 1995 John Wiley & Sons, Inc.