The development of brain and behaviour is controlled by the interaction of genetic determinants and environmental factors. To study genetic determinants, model systems such as the Naples rat lines, i.e. Naples high (NHE) and low excitability (NLE), are useful. They have been selectively bred for divergent behaviour arousal to novelty. Aim of this study was to assess the extent of the genetic control of the selection trait. Thus adult albino rats of NHE and NLE lines have been used throughout. According to a classical Mendelian cross-breeding design, a first experiment was carried out with hybrids obtained from parental lines P1 (NHE) and P2 (NLE) as F1, F2 and related backcrosses B1 (F1xP1) and B2 (F1xP2). Young adults (60-80 days) offspring of both gender were exposed separately for two 10min tests to a spatial novelty (Lát-maze). To verify a possible sex link of the trait, a second experiment was carried out adding to the Mendelian cross design parental gender. Behavioural variables were horizontal (corner-crossings: HA), vertical (rearings on hindlimbs: VA) or total activity (HVA: HA+VA) scores. The heritability of HVA trait was estimated across the 20 generations of selection and Mendelian cross hybrids. Quantitative-genetic analysis on this trait and its HA and VA components, was applied by the Lynch and Walsh joint-scaling test procedure to evaluate underlying genetic mechanism. The correlation between experimental data of hybrids and estimated values from different heritability models were also computed. Results indicated that (i) the activity scores by Mendelian hybrids were intermediate between the two parental lines and were also graded; (ii) there was no sex effect on the heritability of trait but only a general tendency of females to higher activity levels; (iii) the heritability of HVA trait was very high (h2 index=0.824); (iv) heritability model of HVA and HA trait was polygenic with a marked epistatic control where as VA trait was fitted by simpler model with less genes and lower epistatic effect. In conclusion the Naples lines reveal strong genetic determinants for behavioural traits associated with polygenic pattern. Moreover, HA and VA activity components with prevailing cognitive and non-cognitive meaning, respectively, show differential genetic control.