In 110 normal infants at the end of their 2nd, 3rd, 4th and 5th month of life a conditional orienting head turning to a sound reinforced by the presentation of the mother, a stranger or different toys was established. The change of the percentage of conditional responses in the course of 10 sessions was a measure of the conditioning process and of the reinforcing strength of the stimuli. A stimulus known from other natural situations, e.g. the mother, but always used in the experimental situation in the same manner as a reinforcer for the establishment of conditional orienting responses has a poor reinforcing effect. The reinforcing effect of an unknown stimulus is greater notwithstanding it is always used in the same manner (a stranger, the same toy). A new always changing stimulus (different toys) has the greatest reinforcing influence on the conditioning of orienting reactions.