Angiotensin I-converting enzyme (ACE) inhibitory and hypotensive effects of 7 peptide fractions (Frs) of royal jelly protein hydrolysate (RJPH) were studied in comparison with those of RJPH alone. Fr 4 and Fr 5 were the highest in ACE inhibitory activity and yield, respectively. Molecular weights (MWs) of RJPH and Fr 1-Fr 7 were distributed from 100 to 5,000 and those of Fr 1-Fr 7 increased in order from Fr 1 to Fr 7. RJPH, Fr 3 and Fr 4 at doses of 10, 30 and 100mg/kg i.v. and Fr 5 and Fr 6 at doses of 30 and 100mg/kg i.v. caused transiently significant hypotensive effects in spontaneously hypertensive rats (SHR). Fr 3, Fr 4, Fr 5 and Fr 6 at a dose of 1,000mg/kg also caused significant hypotensive effects 3h, 4-5h, 7-8h and 8h after oral administration in SHR, respectively. RJPH caused a long-lasting hypotensive effect in proportion to the magnitude of the MWs of RJPH fractions. The hypotensive pattern of RJPH was similar to the combined pattern of Fr 3-Fr 6. From these results, it can be concluded that the long-lasting hypotensive effect of oral administration of RJPH is dependent on the MWs of its ACE inhibitory peptides and the time required to digest them.