Gloxinia (Sinningia speciosa Lodd.) seedlings were grown in 2 media to determine the maximum percentage N that can be supplied as NH₄ and the minimum concentration of a soluble complete fertilizer that can be applied without reducing plant quality under different light and temperature conditions. In one group of experiments, plants were supplied modified Hoagland's solutions in which the N source and level were varied. Shoot dry weight of gloxinias grown in an organic medium in the winter was lower with 100% NH₄ than with 100% NO₃ but was unaffected by N level, while in the fall shoot dry weight was lower with 100% NH₄ than with 0 or 50% NH₄, and was reduced at 16 rather than 8 meq N liter⁻¹. In the summer, shoot dry weight did not vary with N source or concentration when treatment solutions were collected and reused for 1 week. Flower bud count at harvest did not vary with N source in the winter but was reduced at 100% NH₄ in the fall and summer. Time to flower bud set was unaffected by N source, but was increased by applying 16 rather than 8 meq N liter⁻¹. In the winter, leaves of plants receiving 75-100% NH₄ were dark green early in the studies, but became chlorotic, twisted and quilted before plants matured; in the summer and fall these symptoms of NH₄ toxicity were absent. Leaf chlorophyll content was greater for plants grown in quartz sand when given 100% NH₄ rather than 100% NO₃. In related studies, gloxinias were treated at every watering with a soluble 20N-8.7P-16.7K fertilizer. Shoot dry weight was greater and time until flower bud set shorter at 200 than at 0, 400 or 600 ppm N in the summer; shoot dry weight and flower bud count were greater at 100 than at 200-400 ppm N in the winter. Days until flowering in the summer and days until bud set in the winter were unaffected by fertilizer levels. Tissue composition varied with N source and level and Ca(OH)₂ treatment although lime application did not affect any external plant characteristics. / Ph. D.