Iron-deficient (5 ppm Fe) or control (307 ppm Fe) diets were fed ad libitum to female rats for 7 weeks, and then meal-fed for 4 weeks. Body weights, hemoglobin levels, and hematocrits were lower (p less than 0.01) in deficient group (184 +/- 7, 7.1 +/- 0.4, 32.7 +/- 0.6) than in the control group (220 +/- 10, 16.9 +/- 0.3, 51.8 +/- 0.8) at the end of the 11-week experiment. Animals were killed 4 hr after meal feeding, and liver slices, mesenteric adipose tissue, and segments of mid-jejunum were incubated in vitro with [U-14C] glucose or 3H2O. Adipose tissue from deficient rats had incorporation of [U-14C] glucose into triglycerides two to three times greater than control rats (p less than 0.01). Release of 14CO2 from glucose was greater in adipose tissue of deficient rats than controls (p less than 0.05). Incorporation of 3H2O into triglycerides was also two to three times greater in deficient adipose tissue than in controls (p less than 0.02). In liver slices, incorporation of glucose in polar lipids was slightly higher in deficient rats than in control rats (p less than 0.05). No significant differences were found in incorporation of 3H20 or [U-14C] glucose into lipids or CO2 in jejunum. Thus, iron- deficient adult rats have greater lipid synthesis from 3H2O and glucose in adipose tissue than rats fed adequate levels of iron.