A deepening in the biological nature of the general factor of personality (GFP) is suggested: the activation level of the stress system is here represented by the gene expression of c-fos. The results of a single case experimental design are reported. A model of four coupled differential equations that explains the human personality dynamics as a consequence of a single stimulant drug intake has been fitted to psychological and biological experimental data. The stimulant-drug conditioning and its adaptation to the considered mathematical model is also studied for both kinds of measures. The dynamics of the c-fos expression presents a similar pattern to the dynamics of the psychological measures of personality assessed by the GFP-FAS (Five-Adjective Scale of the General Factor of Personality) as a consequence of a single dose of stimulant drug (methylphenidate). The model predicts similar dynamic patterns for both psychological and biological measures. This study proves that describing mathematically the dynamics of the effects of a stimulant drug as well as the effects of a conditioning method on psychological or subjective variables and on gene expression is possible. It verifies the existence of biological mechanisms underlying the dynamics of the General Factor of Personality (GFP).