Alysia manducator; ENDOCRINE-SYSTEM; Neurosecretory cells role in larval development at various temperatures; LARVAL-DEVELOPMENT; Temperature & neurosecretory cell activity relationships; TEMPERATURE-; Larval development & neurosecretory cell activity relationships Photoperiodic response of Alysia manducator on the daily temperature rhythm have been compared with the state of neurosecretory system of the insect. Data on the reactions at constant temperature and thermorhythms 12-27 degree and 20-27 degree at the photoperiod of LD 12:12 are reported. Length of the high temperature (thermophase) is 4 h in all variants, thermophase was combined with 6 different parts of the photoperiod. Combination of the thermophase with 3 different periods of the scotophase at the temperatures of the cryophase 12 degree or 20 degree produces a similar reaction: active development (without diapause) occurs when thermophase is linked with the first 4 h or last 4 h of the scotophase, and the diapause is observed when thermophase is linked to the middle of the scotophase. Combination of the thermophase with 3 different periods of photophase gives the proportion of the diapausing individuals depending on the position of the thermophase and the cryophase temperature. Neurosecretory system states were compared only under LD 12:12 at constant temperature 20 degree and at daily temperature cycle 20-27 degree when thermophase was linked with the last 4 h of the scotophase or thermophase was linked with the middle of the scotophase. Histological state of the neurosecretory system of brain at constant temperature 20 degree most closely corresponds to state of the neurosecretory system when thermophase was linked with the last 4 h of scotophase. In both cases active development was observed. When the thermophase was linked with the middle of the scotophase, diapause and the corresponding state of the neurosecretory system were observed. These experiments support the hypothetic existence of two antagonistic centres of regulation of the development of insects (Zaslavsky, 1984, 1988). It is suggested that the median neurosecretory cells play a role of hypothetic activating centre and lateral group of cells play a role of inhibiting centre.