The search for the molecular bases of neuro-behavioural traits in Spontaneously Hypertensive Rats (SHR), an animal model of Attention Deficit Hyperactivity Disorder (ADHD), led to the discovery of two quantitative trait loci related to the locomotor activity in the centre of the open field. In the present study, rats from an F2 intercross between the SHR and Lewis strains were selected with markers on the basis of their genotype at these two loci. We obtained a 'high line' in which rats have the alleles increasing the trait, and a 'low line' with the lowering alleles. In activity cages with a dim light, the low line was more active than the high line. The reverse was found in the open field, and the inhibition of locomotor activity in the low line (as compared to the high line) was directly related to the aversiveness of the situation (larger in the centre than in the periphery, and in high light than in low light), and was more intense in males than in females. This inhibition is not attributable to a classical 'anxiety' factor as measured in the elevated plus maze, in which the open arms behaviours were not different between the lines. The high line also showed a deficit in prepulse inhibition of the acoustic startle reflex. The present data show that the two loci previously described in a SHR x Lewis intercross as related to the activity in the centre of the open field are indeed involved in a behavioural inhibition trait. The marker-based selected lines described here are unique tools for the study of the neurobiological bases of this trait and the molecular foundations of its variability of genetic origin.