The role of the dopaminergic and serotonergic system was studied during the embryonic development of the pond snail Lymnaea stagnalis, with special attention to the effect of dopamine and serotonin as well as their agonists and antagonists on the rotation of the veliger larvae, and to the effect of precursors and inhibitors of the synthetizing enzymes on the duration of the embryonic life. Serotonin, D-lysergic acid diethylamide and N,N-dimethyltryptamine increased at a concentration of 1 mM the rotation by 50%, 90% and 87% respectively, and among them D-Lysergic acid diethylamide was found to be the most potent agonist. Other serotonergic agonists and antagonists enhanced the frequency of the rotation (from 165% to 355%) at higher threshold concentrations in the following rank order: methysergid?tryptamine? 2,5-dimethoxy-4-iodoamphetamine ? 5-carboxyamidotryptamine ? bromo-lysergic acid diethylamide ? 7-methyltryptamine. Application of 1-(2-methoxyphenyl) piperazine decreased the rotation by 76%. The reuptake inhibitor desipramine completely blocked the rotation and killed the embryos. Dopaminergic agonists accelerated the rotation by 62% to 233%, and their effect was ranged as follows: dopamine ? apomorphine ? m-tyramine ˜ p-tyramine. Chlorpromazine at 100 µM concentration killed the embryos. At a concentration of 100 µg/ml, tyrosine, the precursor of DA, slowed down the embryonic development by increasing the duration of the embryonic life from 8 to 10 days. Decarboxylase inhibitors, a-methyl-3,4-dihydroxyphenyl-alanine (25 µg/ml) and m-hydroxybenzylhydrazin (5 µg/ml), killed 50% of the embryos, meanwhile the rest hatched ten days later, compared to the control animals. The development was partially blocked by the serotonin precusor L-tryptophane (50 µg/ml). Trytophan hydroxylase blocker, p-chlorphenylalanine (50 µg/ml) resulted in a distortion of the body pattern of the embryos, and prevented the hatching of most (95%) of the animals.