The effects of taurine on the behaviour of male mice were investigated using operant conditioning task. This amino acid is found in high concentrations in the central nervous system of mammals and human maternal milk, has been shown to be essential for the development. Male mice (C57BL/6) were used as subjects and divided into four groups, 1) Long-loading: Taurine (400 mg/kg in a day) dissolved in distilled water was provided as drinking water. In the prenatal period, the taurine was given to through subjects' mothers, from the birth to weaning period, through maternal milk and after the weaning, from the drinking water by the end of the experiment. 2) Loading in pre-weaning: subjects were exposed to the taurine by the weaning. 3) Loading in adult: from after the weaning. 4) Control: no loading (drinking distilled water). Subjects were trained for reacting to illuminated key as soon as possible. As indexes of learning, correct reaction ratio and reaction time was subjected for analysis.All subjects acquired this discrimination task in one month period of the training. The learning speed of each experimental group was different depending on the timing but not length of the loading. [J Physiol Sci. 2006;56 Suppl:S209]