Locoweed (Oxytropis sericea) was fed to 4 open cycling cows that had repeatedly consumed locoweed in previous grazing trails. They received locoweed at 20% of their diet for 30 d (0.68-0.76 mg swainsonine/kg/d). Locoweed induced an immediate rise in serum swainsonine (the locoweed toxin) and a concomitant drop in serum alpha-mannosidase activity in all cows accompanied by abnormal estrus behavior, increased estrous cycle lengths and failure to conceive. Serum progesterone (P4) profiles demonstrated that estrous cycles lengthened from an average of 19 d before locoweed feeding to an average of 34 d (range 24-43 d) while on locoweed. After locoweed feeding ceased, normal estrous cycles returned within an average of 14 d (range 7-25 d). Two of the 4 cows conceived on their first post-locoweed estrus at 7 and 25 d. The third cow bred twice at 13 and 31 d after lowoweed feeding stopped, and the fourth cow bred 3 times at 11, 31 and 52 d before conception occurred. Pregnancies in all 4 cows progressed normally to 7 mo gestation when locoweed was again fed at 20% of the diet for 40 d (gestation days 213 and 253) to 2/4 cows, 1 of which aborted 10 d after lowoweed feeding stopped (263 days gestation). The other cow fed lowoweed calved normally as did the 2 pregnancy control cows. Serum P4 and estradiol (E2) profiles during pregnancy appeared normal before, during and after locoweed feeding except in the cow that aborted, whose P4 declined and E2 increased prematurely. The general trend of serum prolactin was similar in locoweed-fed and control cows.