The developmental change in rat taste sensitivity during the period from the 2nd to the 12th postnatal week was studied by recording the responses of the chorda tympani nerve to lingual stimulation by NaCl, LiCl, KCl, NH4Cl, HCl, quinine hydrochloride (Q-HCl), and sucrose. The threshold for all stimulants except sucrose was lower in the 3-week-old rats than in the 12-week-old ones. The order of the magnitudes of the normalized neural responses to the 0.1 M monochloride salts of the gustatory nerves was NH4Cl greater than NaCl = LiCl greater than KCl in the 3-week-old rats, and NaCl = LiCl greater than NH4Cl greater than KCl in the 12-week-old rats. The magnitude of the normalized responses to NH4Cl, HCl, and Q-HCl at nearly all concentrations was greater at 3 weeks than at 12 weeks, while the magnitude of the responses to NaCl and LiCl at high concentrations was lesser at 3 weeks than at 12 weeks. Changes in the gustatory sensitivity to various taste stimuli during development can be classified into the following two types: (1) sensitivity-decreasing type--the magnitude of the response to NH4Cl, Q-HCl, and HCl was greatest during the first few weeks and decreased thereafter, and (2) sensitivity-increasing type--the magnitude of the response to NaCl, LiCl, and sucrose showed an increase accompanying the growth of rats from around the 2nd week after birth.