Membrane transporters are crucial for mantaining homeostasis and metabolism, especially during the early development of an organism. Organic Cation Transporters (OCTs), members of the Solute Carrier (SLC) superfamily, are mostly localized on the cell membranes of kidney and liver, organs crucial for biotransformation and xenobiotic secretion. The aim of this study was to investigate the activity of membrane transporters of zebrafish (Danio rerio) embryos and larvae, with emphasis on the membrane transporter Oct1, member of the Slc22 family. In addition to wild-type zebrafish, those deficient for the Oct1 transporter, obtained by establishing and genotyping generation lines of genetically modified mutants, were also used. The activity of membrane transporters was observed by in vivo monitoring of the accumulation of fluorescent compounds in zebrafish – their uptake into the organism, accumulation in organs and excretion into the environment, while the activity of Oct1 transporter was monitored by using its supstrates. In addition to these in vivo methods, the activity of the Oct1 transporter was investigated also by in vitro immunofluorescence analyzes of intracellular and tissue expression, and in silico analysis of the expression profiles of membrane transporters by embryonic and larval stages. The results show that the Oct1 transporter is located in the kidney of the zebrafish larva, where it transports ASP+. It was also concluded that Oct1 did not affect the viability and fertility of zebrafish of the observed ages. Using zebrafish throughout all experiments, benefits of zebrafish as a model vertebrate organism for genetic and ecotoxicological research have been demonstrated.