Relaxin is an ovarian polypeptide hormone which is present in large amounts in the rat during the second half of gestation. During this period, blood pressure declines markedly, especially in spontaneously-hypertensive rats (SHR). To test the hypothesis that relaxin might be implicated in this decrease in blood pressure, we infused the hormone in female non-pregnant rats by means of an osmotic mini pump. Our results show that intravenous infusion of purified rat relaxin (1.8 micrograms/day) markedly reduced systolic blood pressure for at least 5 or 6 days in SHR. This decrease was highly significant from 24 hours after the beginning of the infusion and remained significant after 5 days. Rat relaxin was ineffective in control Wistar-Kyoto rats (WKY). Infusion of purified porcine relaxin (3.0 micrograms/day) also diminished blood pressure in SHR, but the effect was less pronounced and developed more slowly, reaching statistical significance on the fourth day of infusion. SHR not receiving relaxin maintained their original systolic blood pressure throughout the experiment. These results indicate that relaxin is involved in the regulation of blood pressure during gestation.