3D spheroids of primary human hepatocytes (3D PHH) keep a differentiated phenotype with retained metabolic function and proteome fingerprint for weeks in culture. As a result, 3D PHH are gaining ground as a model for mechanistic studies on liver homeostasis and in vitro to in vivo (IVIV)predictions in drug discovery. However, the function of drug-transporting proteins has not yet been studied in the 3D PHH. Here we used the organic cation transporter 1 (OCT1/SLC22A1) as a model to explore both transporter kinetics and long-term regulation of transporter activity via different pathways. The 3D PHH were cultured for one week and then used for short- and long-term studies. The OCT1 transporter kinetics was assessed using the fluorescent model substrate 4-(4-(dimethylamino)styryl)-N-methylpyridinium (ASP+) and known OCT1 inhibitors in individual 3D PHH. For long-term studies, the 3D PHH were exposed to xenobiotics for seven days, after which protein expression and OCT1 function were assessed. Global proteomics analysis was used to track prototypical changes of other regulated proteins. ASP+ kinetics indicated a fully functional OCT1 transporter with a Km value of 14.14±3.97 using three donors. Established OCT1 inhibitors reduced the uptake of ASP+ in the 3D PHH spheroids, resulting in 40-60% of the maximal uptake rate of ASP+. The long-term exposure studies showed that OCT1 is relatively stable to the activation of a broad range of pathways known to regulate ADME proteins. Importantly, while expression levels of other ADME proteins, such as CYP3A4 and MDR1 were regulated as expected, OCT1 levels remained stable. In conclusion, our results show for the first time that 3D PHH spheroids express fully active OCT1 and that transporter kinetics can be studied in 3D PHH. We also confirm that OCT1 remains stable and functional during activation of various pathways that alter the expression and function of other ADME proteins.