The experiments proposed herein constitute an ongoing investigation into the genetic control of pyrimidine biosynthesis. We initiated this investigation with experiments geared towards developing our understanding of the physiology of pyrimidine synthesis in the wild type. These experiments were specifically designed to serve as background so that we might be able to clearly recognize genetic variants in which this physiology is altered in some way. In the present proposal, we outline three specific sets of experiments which will enable us to identify genes, and genetic regions which play some fundamental role in regulating pyrimidine synthesis. All of these experiments involve the manipulation of the rudimentary locus of Drosophila, which appears to contain the structural genes for the first three enzymes of pyrimidine synthesis. In the first experiment we propose a positive screen which will allow the efficient recovery of chromosomes that are broken at, or very near to, the rudimentary locus. The rationale of the experiment is that a proportion of these breaks will separate the structural part of the rudimentary locus from some or all of its regulatory elements and we will investigate the physiological consequences of this event. In our second proposed set of experiments, we will employ a sensitive screen which we have devised for the detection of rather minor increases or decreases in enzyme activity in two specific tissues, the wing imaginal disc and the ovary. This screen is specifically designed to enable us to detect mutants with tissue-specific alterations. Our third set of experiments will allow us to detect genes, other than those at the rudimentary locus, which play some fundamental role in the biosynthesis of pyrimidines.