Microglial activation has been considered as a result of neuronal damage, however, recently it becomes to recognize as a possible cause of the damage in various neurodegenerative diseases. To elucidate the mechanism of the microglial activation, we examined the time course of lipopolysaccharide (LPS)-induced change in morphology and the release of cytokines and nitric oxide (NO) in cultured microglia from neonatal rat brains. With addition of 1 microg/ml LPS, the cell morphology was drastically changed within 3 h from amoeboid shape to bipolar rod shape. The peak time of such morphological change was at 6 h and then returned to small round shape gradually. This transient change in morphology was completely inhibited by 0.1 mM dibutyryl-cAMP. On the other hand, the release of cytokines and NO showed different time courses after stimulation by LPS; at first tumor necrosis factor (TNF)-alpha was released within 1 h lag time, secondly interleukin (IL)-1beta within 3 h, thirdly IL-6, and at last NO was released with about 6 h lag time. The addition of dibutyryl cAMP markedly inhibited the release of TNF-alpha and IL-1beta, but not IL-6 and NO at all. These results suggest that there are at least two different intracellular signaling pathways of LPS-induced microglial activation; one for early release of TNF-alpha and IL-1beta sensitive to dibutyryl-cAMP and the other for late release of IL-6 and NO insensitive to dibutyryl-cAMP. The transient morphological change seems to be associated with the early release based on the sensitivity to dibutyryl-cAMP.