Based upon the promising results of previous libraries that incorporated a thioamide/amide "switch", we have prepared a new library of thiorhodamine analogues in order to more closely examine the effects of isolated structural modifications on P-gp activity. Biological examination of these thiorhodamines suggested that P-gp expressing cells exhibit "selection rules" with respect to incorporation of amide and/or thioamide functionality; rates of transport by the pump for amide-bearing derivatives were more rapid than for thioamides. The results of this library suggested that appropriate functionalization of rhodamine dyes could allow one to tune their in vivo properties to be desirable for a particular application. To this end, we have prepared several libraries of selenorhodamine dyes and examined them for their ability to function as photosensitizers in the photodynamic therapy of malignant cells, as well as in the selective depletion of alloreactive T cells in extracorporeal photopheresis. Thioamide-bearing dyes were found to be effective photosensitizers in photodynamic therapy (PDT), whereas amide-bearing dyes showed desirable activity in extracorporeal photopheresis. Recently, the use of tellurorosamine dyes as reversible fluorescent probes for the continuous monitoring of reactive oxygen species in vivo was reported. Such probes are based on the Te(II)/Te(IV) redox cycle. We have evaluated several tellurorosamine dyes for their potential use as reversible probes, explored various methods of their oxidation, and examined their chemical stabilities under cell-like conditions. Oxidized analogues of tellurorosamine dyes in which the 9-phenyl substituent has been functionalized with ortho-methyl groups display increased hydrolytic stability relative to those bearing solely hydrogens at ortho-positions. Our original examination of thiorhodamine dyes and their effects on P-gp activity also showed that dyes having fused ring moieties at the 3-amino position resulted in decreased ATPase stimulation and increased binding affinity relative to those having dimethylamino fragments at the 3-amino position. Hence, we have developed synthetic routes towards rhodamine dyes having fused rings at both the 3- and 6-amino positions in an effort to develop novel, potent inhibitors of P-gp. Initial results suggest that selenorhodamines having fused ring moieties at both the 3- and 6-amino positions are more effective photosensitizers in PDT of cancer cells than those having fused rings only at the 3-amino position. Furthermore, oxidized analogues of tellurorosamine dyes functionalized with fused rings at both the 3- and 6-amino positions display increased hydrolytic stability relative to the analogous 3,6-bis(dimethylamino) tellurorosamine derivatives.