1. The potent bronchoconstrictors prostaglandin (PG) D2, PG F2 alpha and thromboxane A2 are thought to have a role in the pathogenesis of asthma, mediated via the thromboxane (TP) receptor. 2. BAY u 3405 is a new potent selective competitive TP receptor antagonist. 3. The effect of single oral doses of 20 mg and 50 mg BAY u 3405 was examined against histamine and PG D2 bronchial provocation at 90 min after drug ingestion and, for the 20 mg dose alone, at 60 min after ingestion, in randomised, double-blind placebo controlled crossover studies. A time course study was performed with the 20 mg dose. 4. BAY u 3405 protected against PG D2 bronchial provocation. The 20 mg dose increased the amount of PG D2 required to produce a fall of 20% in the forced expiratory volume in 1 s by 6-fold and 16-fold at 60 min and 90 min after ingestion respectively, and the 50 mg dose by 14-fold at 90 min after ingestion. 5. The specificity of the drug was confirmed in vivo in that there was no significant protection against histamine bronchial provocation at either dose or at either time point. 6. The time course study showed significant protection against PG D2 bronchial provocation at 1 h and at 3 h after a single 20 mg oral dose. 7. There was no correlation between subjects in plasma BAY u 3405 concentration and drug effect. Within the subjects performing the time course study there was a strong correlation in time between drug effect and plasma BAY u 3405 concentration.(ABSTRACT TRUNCATED AT 250 WORDS)