The enzyme activity and activation energy of plasma beta-N-acetylhexosaminidase (Hex) was determined in rats whose serotonergic system had been pharmacologically altered. In the group of animals treated with 5-hydroxytryptophan, in the different dissected brain regions (brain stem, cortex and hippocampus) significantly higher levels of serotonin and 5-hydroxyindolacetic acid were found, and significantly lower in the group treated with p-chlorophenylalanine, than in the control group. In the total number of animals studied (n = 21), a statistically significant correlation was found between the plasma concentration of 5-hydroxyindolacetic acid and the levels of this metabolite in the different brain regions (p < 0.001). No significant differences were found for the activity of Hex in the plasma, or for its activation energy, which is a marker of its isoenzyme composition, among the three groups of animals. The results obtained using our experimental model in rats do not confirm the hypothesis of other authors who suggest that the Hex responds secondary to increases or decreases of serotonin turnover, and could be a biological test to monitor the serotonin status in psychiatric patients.