Perinatal mortality is high among small-for-gestational age (SGA) piglets and continues to be an economic burden and threat to animal welfare. As the physiological role of serotonin (5-hydroxytryptamine, 5-HT) in perinatal development and gastrointestinal function in the pig remains unknown, the aim of this study was to assess the enteric distribution of 5-HT cells and to determine 5-HT together with its precursor tryptophan in the serum of perinatal normal and SGA piglets. For this purpose, proximal and distal parts of the small intestine (SI) were processed for immunohistochemical analysis to assess the presence of 5-HT endocrine cells. Serum 5-HT was measured with ELISA, whereas its precursor, that is, the free fraction of tryptophan (FFT) together with albumin-bound tryptophan and total tryptophan, were analysed with HPLC in postnatal piglets. In addition, the morphological growth patterns of the different intestinal tissue layers of both normal and SGA piglets were stereologically analysed. The stereological volume density of 5-HT enteroendocrine cells showed a significant interaction effect between age and region. Indeed, the amount of 5-HT cells in both the proximal and distal part of the SI tended to decrease according to age, with the lowest values detected at day 3 postpartum. No differences could be observed related to BW. Interestingly, the serum concentration of 5-HT was higher in normal piglets compared with SGA piglets. Moreover, the ratio of FFT to total tryptophan was significantly affected by age and BW. Normal piglets had, on average, a lower FFT/total tryptophan ratio compared with SGA piglets. An approximate linear decrease was observed with increasing age. Finally, the immaturity of the intestinal system of the SGA piglets was not reflected in altered volume densities of the different intestinal layers. To conclude, although no BW effect could be detected in the distribution of enteric 5-HT cells, serum 5-HT and the ratio of FFT to total tryptophan ratio showed significant differences between normal piglets and their SGA littermates.