Boric acid (BORA), an ingredient of many cosmetics, pharmaceuticals, and pesticides, was tested for developmental toxicity in timed-pregnant Swiss mice and Sprague-Dawley rats (n = 26-28/group). BORA (0, 0.1, 0.2, or 0.4% in feed) was provided throughout gestation to attain steady-state exposure as early as possible during prenatal development. Average doses (mg/kg/day) were 248, 452, or 1003 in mice, and 78, 163, or 330 in rats. To limit prenatal mortality, BORA (0.8% or 539 mg/kg/day) was provided to an additional group of rats on Gestational Days (GD) 6 to 15 only. On GD 17 (mice) or 20 (rats), fetuses were weighed and examined for malformations (external, visceral, skeletal). Mouse dams exhibited mild renal lesions (greater than or equal to 0.1%), increased water intake and relative kidney weight (0.4%), and decreased weight gain (0.4%) during treatment. There was a reduction of fetal body weight (greater than or equal to 0.2%) and an increased incidence of resorptions and malformed fetuses per litter (0.4%). Morphological changes included an increased incidence of short rib XIII (a malformation) and a decreased incidence of rudimentary or full rib(s) at lumbar I (an anatomical variation). Maternal rats exhibited increased liver and kidney weights at greater than or equal to 0.2%, altered water and/or food intake at greater than 0.2%, and decreased weight gain at greater than 0.4%. Average fetal body weight/litter was reduced at all doses. Prenatal mortality was increased only at 0.8%. The incidence of fetal malformations was significantly increased at greater than or equal to 0.2%. The most frequently observed malformations were enlarged lateral ventricles of the brain and agenesis or shortening of rib XIII. In rats, the no-observable-adverse-effect level (NOAEL) for maternal toxicity was 78 mg/kg (0.1%), while in mice the low dose of 248 mg/kg (0.1%) approached the maternal NOAEL with mild renal lesions in only 2 of 10 females. Embryo/fetal toxicity occurred in all groups of rats at greater than or equal to 78 mg/kg (greater than or equal to 0.1%) while the NOAEL for developmental toxicity in mice was 248 mg/kg (0.1%). Thus developmental toxicity occurred below maternally toxic levels in rats as well as in the presence of maternal toxicity in mice and rats.