Pregnant ewes and their fetuses were chronically catheterized using aseptic procedures under general anaesthesia, and the ewes were then fed either lucerne chaff alone, or lucerne mixed with dried plant material obtained from one of three forb species, Tribulus terrestris (caltrop), Abelmoschus ficulneus (native rosella) or Ipomoea lonchophylla (cowvine), from 103-112 days gestation until term. Ingestion of the forb material was not associated with changes in maternal blood gases, plasma glucose concentrations, or the length of gestation. However, ingestion of rosella seed was associated with a significantly greater fall of fetal arterial pO2 with advancing gestation, and ingestion of either rosella or cowvine was associated with significantly lower fetal mean arterial pressure at 127-131 days, compared with the Tribulus and lucerne groups. Also, the incidence of fetal breathing movements was significantly lower, and did not show a normal day-night variation, in each of the forb-fed groups compared with the lucerne-fed group. The results indicate that these forb plants may contain substances that affect the functional development of the fetal brain. Although ingestion of these plants did not appear to affect the outcome of pregnancy in this study, the possibility that these forbs have a greater impact in sheep populations with poor nutrition and in more extreme environmental conditions is discussed.