Cotton management often involves field scouting to determine when, where, and how much plant growth regulator (PGR) to apply. This study was conducted to determine the capability of remote sensing to discriminate differences in cotton growth and physiology due to applications of either mepiquat chloride or mepiquat pentaborate ranging from 0 to 36 oz per acre. Remote sensing data comprised (1) airborne multispectral imagery at 540, 695 and 840 nm in 20 nm-wide wavebands using ITD Spectral Visions RDACS system, and (2) in situ hyperspectral leaf and canopy reflectance measured in 5-nm wide wavebands between 350 and 950 nm using GER 1500 Spectroradiometer. Moderate rates of PGR decreased plant height, but did not affect plant spectral properties. For the 36 oz treatment, multispectral data indicated significantly (P<0.05) less reflectance in the near-infrared (NIR) region at 840 nm and lower values for NDVI [(R840-R695)/(R840+R695)], as compared to controls. The 36 oz rate also led to greater chlorophyll, K, Ca, Zn, and Mn in leaves. Except for significant treatment difference in blue wavebands (370-420 nm), discriminate analysis of in situ leaf and canopy reflectance agreed closely with wavebands obtained from multispectral data. Remote sensing was unable to delineate vigorously growing cotton from plants that received 8-16 oz PGR per acre. While wavelengths, and therefore radiances, in the three spectral channels of the RDACS airborne multispectral imagery appeared adequate for differentiating large difference in PGR rate, our data suggest the need to consider additional reflectance wavebands or vegetation indices to detect levels of PGR encountered in the field.