1. For the experiments larvae ofPeriplaneta americana (25-28 mm) were used. The antennae were amputated: (a) at different levels immediately after molting; (b) at the same level at different times after the molt. The rate of metamorphosis was observed at the first molt after the operation. 2. The proportion of adult molts increases with the length of the stump of the antennae if symmetrical amputations were carried out at different levels on the first day after the larval molt. There is a negative linear relation between the percentage of adult molts (probit values) and the calculated intensity of the regeneration processes which decreases with the length of the stump. 3. If the proportion of adult molts (probits) is plotted against the logarithm of the time of amputation a straight-line relation to time is obtained provided the amputation was carried out during the growth and differentiation period of the antenna. The rate of metamorphosis increases with time. 4. The observed behavior is discussed in terms of a mutual relationship between the endocrine system and the developing tissue. It is suggested that the intensity of the developmental processes is the essential variable controlling the level of prothoracic gland hormone which controls the activity of the corpora allata.