Prostaglandin F 1alpha (tham salt formulation) was injected IM into mature beef heifers at the rate of 0, 25, 75, 125, or 250 mg (4 heifers/dose group) on days 2 and 10, or 0 and 25 mg (4 heifers/dose group) on days 1 through 10. Drug effects were evaluated by means of 19 clinical observations, measurements of body weight and 10 organ weights, 15 clinical chemistry measurements, 9 hematologic measurements, 7 urinalysis measurements, and gross and microscopic observations of 27 tissues. Prostaglandin F2alpha administered on days 1 and 10 or daily for 10 days produced little evidence of pharmacologic effects and no evidence of toxicologic effects. Pharmacologic effects included a slight transitory increase in heart rate in all treated groups and slight hyperthermia in the 250-mg group. Failure to produce important toxicologic effects at 250 mg/day and with luteolytic doses occurring around 15 to 25 mg/day demonstrated that the safety factor was in excess of 10 X.