N-type Ca2+ channel alpha(1B)-deficient mice have increased activity (ambulation, repetitive behavior, and rearing combined), suggesting contribution by the N-type Ca2+ channel, localized in the plasma membrane and essential for neurotransmitter release, on motor activity. We evaluated the effect of a 6-wk postweaning period of either individual or group housing on the activity displayed in a novel environment with or without previous habituation. Without habituation, male homozygous alpha(1B) deficient mice showed significantly higher activity than wild-type controls, with no influence of the housing condition. When habituated, hyperactivity was seen in individually housed but not group-housed homozygous alpha(1B)-deficient mice. The results indicate that controlling for housing condition can be important when phenotypically analyzing mutant mice.