To investigate localization of the action on blood pressure in the peptide chain of parathyroid hormone (PTH), we determined the effect of seven fragments of human (h) PTH-(1-34) fragments on blood pressure in anesthetized rats. hPTH-(1-7) and -(28-34) fragments had no effect up to 1 X 10(-6) mole/kg. hPTH-(8-34), -(13-34), -(18-34), and -(23-34) fragments showed significant hypotensive activity. The decrease in the chain length lowered the hypotensive activity. The results indicate that some amino acids in position 23-34 are necessary for the hypotensive activity. This accords with the previous report suggesting that 5 amino acids in position 24-28 are essential. In addition hPTH-(8-34) and -(13-34) fragments showed initial transient hypertensive activities. hPTH-(1-13) fragment had only hypertensive action, suggesting that some amino acids in position 8-13 are necessary for hypertensive activity.