In order to understand the role of fish gill ventilation in ammonia excretion, in my Ph. D. study, ventilation and plasma ammonia levels will be manipulated experimentally by several different approaches in order to increase or decrease gill water flow while simultaneously measuring ammonia excretion, both before (control) and after ammonia-loading (treatment). Recently I have collected some preliminary data indicating that the hypothesis is correct, at least in rainbow trout. The first goal of my Ph.D. will be to conduct further experiments to confirm the hypothesis in trout. A second goal will be to understand at the whole animal level how ammonia signals are integrated with the O2 and CO2 signals which also control ventilation, especially under natural circumstances (e.g. post-feeding, post-exercise) where all the signals may change. A third goal will be to use neurophysiological and molecular approaches to examine the mechanisms by which ammonia might be sensed in the NECs and in the brain, the relative importance of these two receptor sites, and interactions with the O2 and CO2 pathways. A final goal will be to broaden the study to other fish, such as air-breathing lungfish, primitive lamprey and hagfish, and urea producing elasmobranch (which produce mainly urea rather than ammonia) to try to understand the phylogeny of ventilatory ammonia sensitivity.