An uptake of radioactivity from (plus or minus)-3H-isoprenaline and its inhibition by drugs which potentiate the responses to (minus)-isoprenaline in the guinea-pig isolated trachea.

1. The uptake of radioactivity derived from (+/-)-(3)H-isoprenaline by the guinea-pig isolated trachea has been measured, and the assumption is made that all the radioactivity is due to isoprenaline.2. 40% of the total (+/-)-(3)H-isoprenaline taken up was loosely bound to the tissue while 25% was firmly bound.3. The firmly bound component of the uptake was more susceptible to inhibition by drugs than the loosely bound component.4. Desipramine and cocaine did not reduce the accumulation of firmly bound isoprenaline.5. Cooling to 23 degrees C, guanethidine and phentolamine caused a moderate reduction in the accumulation of firmly bound isoprenaline.6. Phenoxybenzamine and (+/-)-metanephrine caused a highly significant reduction in the accumulation of firmly bound isoprenaline.7. These findings are discussed in relation to previous studies of the uptake of isoprenaline and of other processes which may be related.8. The inhibition of uptake by the agents examined correlated with their potentiation of the action of (-)-isoprenaline found previously.9. It is suggested that a tissue uptake can significantly modify the pharmacological response to isoprenaline in vitro.