Exposure time and magnetic-pole-dependent physiological response of a cyanobacterium, Anabaena doliolum, were studied by exposing the samples to two poles [north (N) and south (S)] of a 0.3 Tesla permanent magnet for 1-6 h. Study revealed that both magnetic poles, N and S, produce different effects, depending on the length of exposure. However, physiological response of the cyanobacterium to N + S mixtures was significantly different from the response to either of the poles, N or S, suggesting a change in structural chemistry of the water or nutrient solution responsible for the magnetobiological effects.