<b><i>Background:</i></b> Investigations into neurochemical mechanisms of opioid addiction are difficult due to the complexity of behavior and multiplicity of involved neurotransmitter and hormonal systems. The aim of this study was to examine the benefits of structured analysis of these mechanisms using the framework of the neurochemical model Functional Ensemble of Temperament (FET) and the example of maternal behavior under the condition of opium consumption in pregnancy. The FET differentiates between (a) endurance, (b) speed of integration, and (c) emotionality aspects of behavior suggesting that these systems are differentially regulated by (a) serotonin-neuropeptides-brain-derived neurotrophic factor (BDNF), (b) dopamine-GABA, and (c) opioid receptor systems, correspondingly. The FET also suggests that mu-opioid receptors (MORs) binding the endorphines (including opium’s ingredient morphine) have a stronger association with regulation endurance, whereas delta-OR have a stronger association with integration of behavior and kappa-OR – with the perceptual mobilization seen in anxiety. To test the predictions of this model, we compared the impact of massive MOR dysregulation on 3 behavioral aspects of behavior and on serotonin, BDNF, and corticosterone levels. <b><i>Methods:</i></b> The study used 24 female white Wistar rats which were randomly divided into (1) control group: pregnant rats without any intervention; (2) opium-exposed group: animals that were exposed to opium during pregnancy and after the delivery until the end of the study. At the end of the study, the levels of BDNF, serotonin (5-HT) in the hippocampus of the mother’s brain, and serum corticosterone, as well as 12 aspects of the maternal behavior were evaluated. The differences between control and experimental groups were assessed using the <i>t</i> test for independent samples. <b><i>Results:</i></b> The BDNF and serotonin concentrations in the hippocampus of the mother rats which were exposed to opium were lower than in the control group; the mean corticosterone in exposed mothers was higher than in the control group. Behaviorally, opium-consuming mothers showed lower endurance in 4 distinct behavioral categories (nesting, feeding, grooming, and retrieval) than the mothers in the control group. Ease of integration of behavior was affected to a lesser degree, showing a significant effect only in 1 out of 5 applied measures. Self-grooming, seen as an emotionality-related aspect of behavior, was not affected. <b><i>Conclusion:</i></b> Opium exposure during pregnancy in our experiment primarily reduced the endurance of rat’s maternal behavior, but the speed of integration of behavioral acts was less affected. This negative impact of opium on endurance was associated with a decrease of BDNF and serotonin levels in the hippocampus and an increase in corticosterone level in opium-consuming mothers. There is no effect of opium exposure on self-grooming behavior. This pattern supports the FET hypothesis about the role of 5-HT and BDNF in endurance, differential regulation of endurance, integrative and emotionality aspects of behavior, and differential association of the MOR system with endurance aspects, in comparison with kappa- and delta opioid receptors.