Stimulation of parasympathetic secreto-vasomotor nerve fibres in cat nasal mucosa produced nasal secretions with biological activity similar to cat paw SRS. The biologically active principle was purified by ethanol extraction, Amberlite XAD-2 and anion exchange chromatography. It had elution characteristics common with cat paw SRS but different to those of prostaglandin F2α. The biological activity on the guinea pig ileum was antagonized by FPL 55712. In the cat intraarterial administration of cat paw SRS resulted in a dose-dependent but modest increase in nasal blood flow whereas local blood content was not affected. Provided SRS has similar effects on human nasal blood flow the importance for SRS as a mediator for nasal congestion in allergic rhinitis or parasympathetically induced vasomotor rhinitis seems less likely.