When environmental temperatures exceed 25°C, horses are potentially subjected to thermal stress. It has therefore been recommended that horses should be provided with shade during hot days. However, this is not possible for horses grazing on many Australian rural properties. Although the positive effect that solar radiation blocking can have on reducing heat absorption is understood by some, conflicting views, mostly anecdotal, exist on the use of a light cotton rug on horses for this purpose. The aim of this pilot study was to examine the effects of wearing a light-colored cotton rug on horse heart rate (HR), respiratory rate (RR), rectal temperature (RT), sweat production, and selected stress-related behaviors. Data were collected for 2 groups of university-owned horses (n = 8 and 10, respectively). The horses were tied in an outdoor arena in direct sunlight for 2 hours on 2 different days (D1 and D2). Baseline behavioral and physiological data (T0) were noted, recording frequency (n/10 min) of tail swishing, licking-chewing, pawing, repeated head movements, and self-care and recording HR, RR, RT, and sweat production using a sweat score (0 = none to 5 = excessive). Half of the horses were then fitted with a light cotton rug, and all horses were observed and monitored at regular 15-minute intervals for a further 2 hours (T1-T8). The effect of repetition (D1 and D2) and time (T0-T8) was not significant; therefore, the data were combined and analyzed using the Mann-Whitney U-test with rug (rugged/unrugged) as the independent variable. RT and sweat score were significantly lower in unrugged horses compared to rugged horses (37.4 ± 0.3 vs. 37.7 ± 0.3°C; 0.5 ± 0.8 vs. 1.9 ± 1.3, respectively; P < 0.001). However, unrugged horses showed a significantly higher frequency of tail swishing and pawing (23.1 ± 25.9 vs. 8.7 ± 11.0 n/10 min; P < 0.001; 9.4 ± 21.2 vs. 5.8 ± 17.4 n/10 min; P = 0.018). Although wearing a rug did not have an effect on the other parameters, it is worth noting that HR, RR, and the occurrence of stress-related behaviors were higher than normal values for equids, suggesting that horses were potentially prone to discomfort. Overall, it appears that the use of light-colored cotton rugs may help reduce the irritation caused to horses by flying insects as evidenced by less tail swishing but may also lead to an increase in internal temperature and subsequently sweat production, increasing the risk of thermal stress and loss of electrolytes. Wearing a rug is not an adequate substitute for the provision of shade when ambient temperatures exceed 25°C.