Summary The efficacy of PGI 2 and 20-methyl-PGI 2 in preventing aspecifically induced bronchoconstriction was evaluated in a single-blind cross-over study in 9 asthmatic patients. PGI 2 and its 20-methyl-isomer were given by inhalation in doses ranging from 250 μg (in women) to 500 μg (in men). Ultrasonic H 2O mist (6 patients) and physical exercise (cycling) were used as bronchial stimulant agents. Both prostacyclins gave an excellent degree of protection but had no direct bronchodilator effect. An increase in heart rate was observed in almost all patients immediately after inhalation of PGI 2 and 20-methyl-PGI 2, but this effect was particularly evident in women in whom a significant decrease in diastolic pressure was noted too.