Testosterone does not only exert its effects through the classical cytosolic and nuclear androgen receptor (AR), whereby this receptor dimerises following intracellular binding of testosterone and is translocated from the cytosol into the nucleus where it binds to the SHRE (steroid hormone response elements) of DNA and influences the transcription of certain genes. Testosterone also brings about non-classical, rapid effects through the membrane-bound ZIP9 receptor. ZIP9 is a zinc transporter from the family of ZRT-/IRT (zinc-regulated transporter/iron-regulated transporter) like proteins (SLC39A) and possesses an extracellular androgen-binding site. These non-classical effects mediated by ZIP9 are of great physiological significance in various different tissues. Testosterone is required for the process of spermatogenesis, for creating and maintaining the blood-testis barrier (BTB) as well as for releasing mature sperm into the seminiferous tubules. Testosterone deficiency leads to infertility in men. Various hormone-based treatments are available for the treatment of testosterone deficiency, including direct supplementation with exogenous testosterone. However, a treatment-associated increase in serum concentrations of testosterone leads to infertility in men due to its effects on the hypothalamic-pituitary-gonadal axis (negative feedback). Adverse androgenic side-effects also result due to effects on the AR. Due to their advantageous naturally-occurring properties, peptides have a great potential for use as pharmaceuticals. The development of synthetic peptides for the treatment of metabolic diseases has been the subject of active research in recent years. Several peptide-based drugs have already been successfully applied in clinical trials for the treatment of various diseases. The aim of this research project was to design peptides in silico that bind to the androgen-binding site of the ZIP9 receptor, and to confirm this action in vitro, as well as to investigate possible intracellular effects. A model of the molecular structure of ZIP9 had already been created as part of a previous research project in 2017 by Bulldan and Malviya from the Scheiner-Bobis working group. This molecular model was revised and further refined over the course of this research project. Six tetrapeptides were also designed in silico and then synthesised, whereby the tetrapeptides were able to bind to the androgen binding site of ZIP9. The actual binding of the peptides to ZIP9 and possible androgenic effects were investigated in vitro on L6 myoblasts, osteogenic SAOS-2 cells, and 93RS2 Sertoli cells. The methods and results from these investigations have been released in two publications (Annexes 1 and 2). Our investigations have demonstrated that all peptides bind to ZIP9 and subsequently bring about androgenic effects in all three cell lines. These results open up the possibility that the peptides we have developed could be used as pharmaceutical substances for the treatment of infertility associated with testosterone deficiency, following further research and development; they could achieve this by exerting direct effects on the BTB whilst at the same time avoiding hormonal effects on the hypothalamic-pituitary-gonadal axis (negative feedback) and androgenic side effects. Further applications for consideration could include promotion of muscle-building and the treatment of bone diseases.