Mice lacking the serotonin receptor 1A (Htr1a knockout, Htr1aKO) show increased innate and conditioned anxiety. This phenotype depends on functional receptor activity during the third through fifth weeks of life and thus appears to be the result of long‐term changes in brain function as a consequence of an early deficit in serotonin signaling. To evaluate whether this phenotype can be influenced by early environmental factors, we subjected Htr1a knockout mice to postnatal handling, a procedure known to reduce anxiety‐like behavior and stress responses in adulthood. Offspring of heterozygous Htr1a knockout mice were separated from their mother and exposed 15 min each day from postnatal day 1 (PD1) to PD14 to clean bedding. Control animals were left undisturbed. Maternal behavior was observed during the first 13 days of life. Adult male offspring were tested in the open field, social approach and resident–intruder tests and assessed for corticosterone response to restraint stress. Knockout mice showed increased anxiety in the open field and in the social approach test as well as an enhanced corticosterone response to stress. However, while no effect of postnatal handling was seen in wild‐type mice, handling reduced anxiety‐like behavior in the social interaction test and the corticosterone response to stress in knockout mice. These findings extend the anxiety phenotype of Htr1aKO mice to include social anxiety and demonstrate that this phenotype can be moderated by early environmental factors.