The present study aimed at investigating the acquisition process of number conservation by 2 training experiments. Ninety-eight 4-6 year-old children, who had been non-conservers at a Pre-test, served as Ss of Exp. I. Elghteen out of them could anticipate correctly changes of the number of a coliection under transfarmations including addition and subtraction and spatial rearran.gement of elements simultaneously. They were divided into a' gr. and b' gr. (Each had 9 Ss) The remaining 80 children could only antici-pate nuinerical changes without spatial rearrange-ment. They were clivided into a, b, c, d and e grs. (Each had 16 Ss) Each 2 children assigned to a' and b gr. did not participate in the training session. Five different training procedures were adopted : Practice in <)onflict situations with external reinfor-cement CSs of a and a' grs. received this method of training), Practice in conflict situations without external reinforcement (b and b''grs. ), Practice in reinforded conservation situations (c gr.), Practice in mixed situations with reinforcement Cd gr. ), and Practice in conflict situations without reinforcement+some auxiliary steps (e gr. ). Each training procedure consisted of 2 sessions of 24 trials, and was given Ss on 2 consecutive days.