Niemann-Pick disease type C (NPC) is a fatal, autosomal recessive lipidosis characterized by a unique error in cellular trafficking of cholesterol. In the disease, unesterified cholesterol as well as sphingolipids accumulate in the late endosomes/lysosomes due to mutations in either of two recently isolated genes, NPC1 or NPC2. A metabolic and neurological disorder reminiscent of human NPC disease has been described in Balb/C mice, and it was recently shown that the mutation in the NPC mice resides in the orthologous murine Npc1 gene. Here we have followed the growth rate and applied behavioural methods in order to establish the onset and development of the major symptoms in the NPC mouse model. Wild type and NPC mice were studied during 28-59 days of age. Both male and female NPC mice displayed retarded growth at the age between 25 and 35 days. At the age of 35-45 days the weight was similar to controls and thereafter very rapidly decreased. The battery of coordination tests (vertical screen, beam balancing, coat hanger and rotating rod) established motor impairment of the NPC mice already at the age of 28-42 days, well before the onset of visually detectable ataxia. Decreased exploratory activity and lack of habituation was revealed in the NPC mice by open field test. The diseased mice were unable to learn and remember the location of the hidden escape platform in spatial water maze task suggesting cognitive impairment. In several tests the male NPC mice were more affected than the females. The present study represents the first behavioural analysis of the NPC mice. The battery of behavioural tests employed here should be valuable in the assessment of effective approaches to treat NPC, for which no preventive or curative measures have so far been established.