There is a significant current need to elucidate the molecular mechanisms of the side-effects caused by widely-used pharmaceuticals. Examples include the acute nephrotoxicity and irreversible ototoxicity promoted by the cationic drugs gentamicin and cisplatin. Gentamicin is an aminoglycoside antibiotic used for the prevention and treatment of life-threatening gram-negative bacterial infections, such as tuberculosis and meningitis. Cisplatin is used to treat a broad spectrum of cancers including head and neck, ovarian, cervical, stomach, bladder, sarcoma, lymphoma, testicular cancer and others. The objective of this study is to design and synthesize rhodamine derivatives that can be used for the construction of geometrically well-defined cationic drug conjugates. The long-term goal is to use the conjugates as tools to aid in elucidating the properties and identities of ion channels involved in the uptake of cationic pharmaceuticals into kidney and cochlear hair cells. This will shed light on the origin and potential prevention of unwanted side effects such as nephrotoxicity and ototoxicity associated with specific cationic drugs. A series of extended rhodamine analogs with reactive groups for biomolecule conjugation has been synthesized. These fluorophores show similar spectral properties to their prototype, Texas Red succinimidyl ester (TR-SE). However, they contain rigid linkers between the fluorophore and amine-reactive moiety. The resultant gentamicin conjugates of these materials are rigidified enabling one to assess channel pore dimensions without the confounding issue of conjugate folding. Preliminary cell studies are promising, as one observes reduced gentamicin uptake in both kidney and sensory hair cell upon systematically increasing the dimension of the fluorophore. This work has enabled us to tentatively assign the maximum dilated MET channel pore size as between 1.44 nm to 1.56 nm. However, this preliminary finding, though encouraging, needs further validation via ongoing studies with larger diameter fluorophore conjugates, A cisplatin-Texas Red conjugate has also been synthesized to enable studies of cellular uptake mechanisms. This conjugate preserves not only the spectral properties of Texas Red after conjugation, but also the cytotoxicity of cisplatin. This has been validated in zebrafish. The series of rhodamine probes that have been conjugated to gentamicin should be similarly useful for cisplatin studies. These studies are planned. Additional future work includes the synthesis of semi-flexible (glycol) and flexible (alkyl) linkers to evaluate structure-activity relationships.