The contractile regulation of neonatal smooth muscle was studied in rat neonatal portal vein. Strips of beta-escin-permeabilized portal vein from 3- to 5-day-old rats and 5- to 6-week-old rats were mounted on a 'bubble chamber' in calcium solutions buffered with 10 mM EGTA. Although the overall tension development was lower in neonatal muscle as expected, the calcium sensitivity of the neonatal permeabilized portal vein was significantly higher than in developed portal vein. Endothelin-1-induced sensitization of the myofilaments was investigated. Endothelin-1 produced an increase in tension of permeabilized neonatal portal vein in a calcium solution buffered with 10 mM EGTA. This sensitization was proportionally higher in neonatal than in developed smooth muscle, despite similar initial submaximal calcium contractions. GTP gamma S-induced calcium sensitization was also proportionally higher in neonatal permeabilized strips than in fully developed smooth muscle. These changes may be due to alterations in the intracellular signalling pathways which mediate calcium sensitization in smooth muscle. As some endothelin-1-mediated responses are known to occur via activation of the heterotrimeric GTP-binding protein, Gq, the levels of protein expression of Gq alpha were studied. In membrane preparations from neonatal rat portal vein the expression of Gq alpha was significantly higher than in portal vein membrane preparations loaded with equal protein from 5- to 6-week-old rats. In conclusion, agonist-induced sensitization of the myofilaments was higher in neonatal rat portal vein than in fully developed portal vein. This difference is the result of changes in intracellular signalling and may be partly produced by the greater expression of Gq alpha observed in neonatal portal vein.