1. The aim of our study was to ascertain the possible differences and/or similarities in natural tachykinin activity in vitro on lower urinary tract of large-sized animals as compared with data obtained in laboratory animals. 2. Besides tachykinins normally present in mammals, namely substance P (SP), neurokinin A (NKA) and neurokinin B (NKB), we tested non-mammalian tachykinins, such as eledoisin (ELED), physalaemin (PHYS), kassinin (KASS) and PG-kassinin II (PG-KASS II). 3. NKA, KASS and ELED were found to be the most potent peptides in contracting detrusor strips from porcine bladder. In particular, NKA showed a pD2 of 7.14, whereas KASS and ELED showed pD2 values of 7.20 and 7.22, respectively. The activity of NKB and PG-KASS II corresponded to 72.4 and 55.0% respectively of that of NKA. SP and PHYS activity corresponded to only 2% of that of NKA. 4. NKA (pD2 7.92) was the most active peptide in contracting bladder neck tissues as well. ELED and KASS were found to have lower, similar pD2 values (7.62 and 7.70, respectively), whereas NKB and PG-KASS II were much less active (pD2 7.12 and 6.74, respectively). Moreover, SP and PHYS showed an activity range lower than 2% of that of NKA. 5. The reported results confirm that, on pig vesical neck and detrusor, NK1 receptors represent a minority as compared with NK2 and NK3 receptors. By contrast, the presence of NK2 receptors is demonstrated by a greater potency of NKA. The presence of NK3 receptors both on detrusor and neck is evidenced by NKB activity and by results achieved with PG-KASS II.