The authors studied the effects of thiopental on baseline airway tone in intact guinea pig tracheas using a preparation where the epithelial (inside) and serosal (outside) surfaces were isolated. Whole tracheas were excised, cannulated, and mounted in 50-ml tissue baths. The serosal and epithelial surfaces were perfusedviaseparate circuits with Krebs-Henseleit solution. All data were expressed as a percent of constriction produced by 2 × 10−6M carbachol (a concentration that elicited a 90 + % of maximal constriction). Thiopental elicited a dose-dependent constriction in all 25 tracheas. Increases in tone were first seen at 10−5M (14.3 ± 1.84%; mean ± SEM) and reached a peak at 10−3M (29 ± 3.16%;P<.0001). Responses to thiopental were similar when the epithelium was removed, when thiopental was added to the inner perfusate, and when tracheas were pretreated with 10−5M pyrilamine. Constriction was entirely inhibited by pretreatment with indomethacin 10−5M. The authors conclude that thiopental, at concentrations in the clinical range, causes a reproducible dose-dependent constriction of guinea pig trachea. This effect is mediated by constrictor prostaglandins.