Today, P-glycoprotein (P-gp) mediated multidrug resistance (MDR) is still a major problem in therapy of cancer and other diseases as epilepsy or dementia. An effective modulator of MDR is outstanding. P-gp plays also an important role in HIV-therapy. A dual mode of action as inhibiting both, HIV target structures, like HIV-1 protease and P-gp, can be advantageous in HIV-therapy. 1,4-Dihydropyridine derivatives combine both characteristics, inhibition of P-gp and HIV-1-protease. Monomeric as well as homo- and heterodimeric structures and 1,4-dihydroquinolines showed potent P-gp inhibiting properties as prooved in FACS analysis and cell assays. A symmetric structure of the dimeric compounds was advantageous for P-gp inhibition, due to the C2-pseudosymmetry of the proteinstructure. Unlike, symmetric homodimers and asymmetric heterodimers showed potent HIV protease inhibition, messured in a fluorimetric in vitro assay. The flexibility of the protease structure allows the structural adaption to the inhibitor molecule. Varied substituents have been introduced to the aromatic residues of the dihydropyridine derivatives. Structure activity relations are discussed.