Burley tobacco undergoes numerous physical and chemical changes during the air-curing process, and quality of the tobacco is dependent on the curing conditions. Because burley tobacco is cured slowly under ambient conditions, little research has been invested in studying detailed effects of environmental conditions on the daily changes in chemical composition during the air-curing process. Our studies were initiated to evaluate degradation of plant pigments and other leaf constituents and accumulation of degradation products during the air-curing process. Changes in the levels of the plant pigments, e.g., carotenoids, are important since they are considered to be precursors for some of the cured tobacco constituents that relate to quality. In 1981, burley tobacco (Ky 14) was grown in field plots under normal burley cultural conditions. Twenty-five days after topping, the tobaccos were harvested and subjected to on-stalk curing in a conventional curing barn and in curing chambers with controlled humidity and temperature. Triplicate samples of leaf were taken at harvest and days 1, 2, 3, 4, 7, 9, 11, 14, 16, 18, 21, 23, 25, 28, 35, and 42. All samples were divided into lamina and midveins, frozen to -40.degree., freeze dried, ground, and stored at -40.degree. until analyzed. Weights of the lamina were taken before and after freeze drying to determine the relative rate of moisture loss during the curing process. Also, leaf plugs were taken for measuring the pigment content on both area and weight bases. Concentrations of chlorophyll-a, chlorophyll-b, beta-carotene, lutein, neoxanthin, and violaxanthin were determined by extraction of the sample with acetone, preliminary purification by use of a reverse phase cartridge (sep-pak), and separated and quantified by reverse phase high performance liquid chromatography with a C-18 column and a detector wavelength of 436 nm. As predicted, results show that the decreases in plant pigments did not follow a linear plo t with respect to time during the curing period. Tobacco cured under ambient conditions in the barn showed relatively little change in pigment concentrations during the first week of curing. After seven days, there was a rapid decrease in the content of all pigments in tobacco samples cured in the barn. Controlled temperature and humidity in the curing chamber gave similar results except that the rapid change occurred between the fourth and seventh days of curing. The differences between barn and chamber curing rates were attributed to ambient weather conditions that affected the barn cure. Data from this study have also shown relative changes of pigment concentration during curing. The carotenoids showed up to a six-fold change in concentration between harvest and final cure while chlorophylls decreased as much as 15-fold in concentration. Data for these changes were adjusted for respiration losses that occurred during curing. This study also included changes in composition of other leaf constituents. In particular, the tobacco samples have been analyzed for some of the non-polar leaf constituents such as neophytadiene and the unsaturated hydrocarbons. Results from this segment of the investigation show an overall increase in concentration of these components. The maximum levels of these components did not occur at the end of the cure but at the time of yellowing (7-10 days after harvest). Data presented from this study show the complexity of the curing process and the results have aided in defining some of the products degraded and formed during the air-curing process.