AbstractHyperpolarizing fast inhibitory neurotransmission by γ‐aminobutyric acid and glycine requires an efficient chloride extrusion mechanism in postsynaptic neurons. A major effector of this task in adult animals is the potassium‐chloride co‐transporter KCC2 that is selectively and abundantly expressed postsynaptically in most CNS neurons. Yet, the role of KCC2 in adult brain at the systems level is poorly known. Here, we characterize the behaviour of mice doubly heterozygous for KCC2 null and hypomorphic alleles that retain 15–20% of normal KCC2 protein levels in the brain. These hypomorphic KCC2‐deficient mice were viable and fertile but weighed 15–20% less than wild‐type littermates at 2 weeks old and thereafter. The mice displayed increased anxiety‐like behaviour in several tests including elevated plus‐maze and were more susceptible to pentylenetetrazole‐induced seizures. Moreover, the mice were impaired in water maze learning and showed reduced sensitivity to tactile and noxious thermal stimuli in von Frey hairs, hot plate and tail flick tests. In contrast, the mice showed normal spontaneous locomotor activity in open field and Y‐maze tests, and intact motor coordination in rotarod and beam tests. The results suggest that requirements for KCC2‐dependent fast hyperpolarizing inhibition may differ among various functional systems of the CNS. As shunting inhibition is expected to be intact in KCC2‐deficient neurons, these mice may provide a useful tool to study the specific functions and relative importance of hyperpolarizing fast synaptic inhibition in adult CNS that may have implications for human neuropsychiatric disorders, such as epilepsy, pain and anxiety.