Voltage-sensitive dyes (VSDs) and optical imaging are useful for studying spatiotemporal patterns of population neuronal activity in cortical tissue. Using a photodiode array and absorption dyes we were able to detect neuronal activity in single trials before it could be detected by local field potential (LFP) recordings. Simultaneous electrical and optical recordings from the same tissue also showed that VSD and LFP signals have different waveforms during different activities, suggesting that they are sensitive to different aspects of the synchronization across the population. Noise, dye bleaching, phototoxicity and optical filter selection are important to the quality of the VSD signal and are discussed in this report. With optimized signal-to-noise ratio (S/N) and total recording time, we can optically monitor approximately 500 locations in an area of 1 mm(2) of cortical tissue with a sensitivity comparable to that of LFP electrodes. The total recording time and S/N of fluorescence and absorption dyes are also compared. At S/N of 8-10, absorption dye NK3630 allows a total recording time of 15-30 min, which can be divided into hundreds of 4-8 s recording trials over several hours, long enough for many kinds of experiments. In conclusion, the VSD method provides a reliable way for examining neuronal activity and pharmacological properties of synapses in brain slices.