Summary. Purified 1R and C, respectively. The calving intervals were reduced in Groups 2R (P < 0\m=.\01) and 1R (P < 0\ m=. \ 05) compared with Group C. The incidence of dystocia was 29% (2 of 7) in Group 2R and 43% (3 of 7) in Group 1R compared with 57% (4 of 7) in Group C (P < 0\m=.\01).Body weights and ratios of males to females of the calves were similar (P > 0\m=.\05) between groups. Progesterone plasma concentrations decreased (P < 0\m=.\01) earlier in Groups IR and 2R compared with Group C, and this acute decrease began within 6 h of treatment. At 24 h after relaxin or PBS injection, progesterone concentrations were 2\m=.\7\m=+-\1\m=.\1 ng/ml for Group 2R, 3\m=.\5\ m=+-\0\m=.\9 ng/ml for Group 1R, and 6\m=.\0\ m=+-\0\m=.\1 ng/ml for Group C. Relaxin reached peak blood-plasma levels of 19 \ m=+-\ 2\m=.\2ng/ml 1h after injection of relaxin, but remained unchanged, 0\m=.\3\ m=+-\0\m=.\01 ng/ml, in Group C. Pelvic area was increased 26%, 22% and 14% and cervical dilatation was increased 109%, 76% and 53% 48 h after injection in Groups 2R, 1R and C, respectively, but these responses were similar among groups at the time of parturition. We conclude that two i.m. injections of relaxin facilitated earlier calving, acutely decreased progesterone secretion, increased cervical dilatation and pelvic area expansion, and decreased the incidence of dystocia in dairy heifers.