A UV laser-based optical microscope and CCD detection system with high sensitivity has been developed to image neurotransmitters in living cells. We demonstrate the detection of serotonin that has been taken up into individual living glial cells (astrocytes) based on its native fluorescence. We found that the fluorescence intensity of astrocytes increased by up to 10 times after serotonin uptake. The temporal resolution of this detection system at 10-4 M serotonin is as fast as 50 ms, and the spatial resolution is diffraction limited. This UV laser microscope imaging system shows promise for studies of spatialtemporal dynamics of neurotransmitter levels in living neurons and glia. © 1995, American Chemical Society. All rights reserved.