Networks of gamma-aminobutyric acid (GABA)ergic interneurons connected via electrical and chemical synapses are thought to play an important role in detecting and promoting synchronous activity in the cerebral cortex. Although the properties of electrical and chemical synaptic interactions among inhibitory interneurons are critical for their function as a network, they have only been studied systematically in juvenile animals. Here, we have used transgenic mice expressing the enhanced green fluorescent protein in cells containing parvalbumin (PV) to study the synaptic connectivity among fast-spiking (FS) cells in slices from adult animals (2-7 months old). We have recorded from pairs of PV-FS cells and found that the majority of them were electrically coupled (61%, 14 of 23 pairs). In addition, 78% of the pairs were connected via GABAergic chemical synapses, often reciprocally. The average coupling coefficient for step injections was 1.5% (n = 14), a smaller value than that reported in juvenile animals. GABA-mediated inhibitory postsynaptic currents and potentials decayed with exponential time constants of 2.6 and 5.9 ms, respectively, and exhibited paired-pulse depression (50-ms interval). The inhibitory synaptic responses in the adult were faster than those observed in young animals. Our results indicate that PV-FS cells are highly interconnected in the adult cerebral cortex by both electrical and chemical synapses, establishing networks that can have important implications for coordinating activity in cortical circuits.