Members of the Na+/Cl- dependent neurotransmitter transporter family perform the re-uptake of neurotransmitter released into synapses and thus control the magnitude and duration of synaptic transmission. The importance of these molecules in the nervous system is further demonstrated by the fact that Na+/Cl- dependent neurotransmitter transporters are targets of psychoactive drugs such as cocaine and Prozac, and are linked to diseases such as Autism. In addition, studies in cultured kidney cells have shown that these molecules also accumulate osmolytes and maintain normal cellular structure and function under hypertonic stress. Inhibition of neurotransmitter transporters in the kidney can cause kidney failure. In this thesis is described the positional cloning of the Drosophila inebriated(ine)/rosA gene that encodes a putative Na+/Cl- neurotransmitter transporter. Mutations in this gene cause increased neuronal excitability at the neuromuscular junction and photoreceptor cells, as well as increased sensitivity to a hypertonic environment. The ine/ rosA gene produces two proteins that differ only in their N-terminal intracellular domain. One form, ine/rosA-l, contains an additional unique sequence of 313 amino acids at the N-terminus compared to the second form, ine/rosA-s. The two transcripts of this gene have very similar distribution patterns in Drosophila embryos. Functional studies on the ine/ rosA gene include various attempts to identify the substrate(s) for the ine/rosA transporter and the localization of the ine/rosA products. Overexpression of the ine/rosA-s cDNA in the Malpighian tubules, the Drosophila analog of mammalian kidney, enhanced Drosophila salt resistance. These studies have established that the ine/ rosA gene plays a vital role in the regulation of both neuronal excitability and water and salt metabolism, and have laid a foundation for further elucidation of neurotransmitter transporter function that may lead to new targets and clues for the treatment of disorders in the nervous system and the kidney.