The hypoxic effects of salbutamol 200 micrograms (CAS 18559-94-9) and bitolterol mesilate 740 micrograms (Biterol, CAS 30392-41-7), both administered by pressurized aerosol, were assessed by transcutaneous pO2 monitoring in 18 asthmatic subjects and in 12 normal volunteers. Unlike the latter, the asthmatics showed a highly significant difference in ptcO2 time course after the two drugs. After bitolterol mesylate the decrease in ptcO2 was much more gradual and the maximal decrease showed a 10-min time lag vs. salbutamol, which may partly explain the well known chronotropic effect of bitolterol. The clinical importance of the beta 2-induced hypoxemia may be related more to the dynamics of the ptcO2 decrease than to its absolute magnitude, which is usually fairly moderate.