In the course of our continuing pharmacological studies of marine microalgae, we look now at Tetraselmis suecica Stewart (Prasinophyceae) and Isochrysis galbana Demart (Haptophyceae), assessing the effects which aqueous extracts of these algae exert upon the central nervous system. To this end, we have tested for effects upon the following: spontaneous motor activity, body temperature, motor coordination, dextroamphetamine-induced hypermotility, exploratory conduct, muscle relaxation, catalepsy, conditioned avoidance response, oxotremorine-induced cholinergic syndrome, and pentylenetetrazole-induced convulsions. Our results show that aqueous extracts of both microalgae display action profiles which prove difficult to categorize by standard pharmacological criteria.