The neocortex has at least two different networks of electrically coupled inhibitory interneurons: fast-spiking (FS) and low-threshold-spiking (LTS) cells. Agonists of metabotropic glutamate or acetylcholine receptors induced synchronized spiking and membrane fluctuations, with irregular or rhythmic patterns, in networks of LTS cells. LTS activity was closely correlated with inhibitory postsynaptic potentials in neighboring FS interneurons and excitatory neurons. Synchronized LTS activity required electrical synapses, but not fast chemical synapses. Tetanic stimulation of local circuitry induced effects similar to those of metabotropic agonists. We conclude that an electrically coupled network of LTS interneurons can mediate synchronized inhibition when activated by modulatory neurotransmitters.