Ethnic differences in insulin secretion and action between African Americans (AAs) and European Americans (EAs) may influence mobilization of free fatty acids (FFAs). We tested the hypotheses that FFA concentrations would be associated with measures of insulin secretion and action before and during a glucose challenge test. Subjects were 48 prepubertal girls, 60 premenopausal women, and 46 postmenopausal women. Fasting insulin (insulin 0), the acute insulin response to glucose (AIR g), the insulin sensitivity index (S I), basal and nadir FFA (FFA 0, FFA nadir), and nadir time (TIME nadir) were determined during an intravenous glucose tolerance test (IVGTT). Stepwise multiple linear regression (MLR) analysis was conducted to identify associations of FFA 0, FFA nadir, and TIME nadir with ethnicity, age group, insulin measures, indexes of body composition from dual-energy X-ray absorptiometry, and measures of fat distribution from computed tomography scan. In this population, insulin 0 and AIR g were higher among AAs vs. EAs, whereas S I was lower, independent of age group. MLR analyses indicated that FFA 0 was best predicted by lean tissue mass (LTM), leg fat mass, ethnicity (lower in AAs), S I, and insulin 0. FFA nadir was best predicted by FFA 0, age group, and intra-abdominal adipose tissue (IAAT). TIME nadir was best predicted by leg fat mass, AIR g, and S I. In conclusion, indexes of insulin secretion and action were associated with FFA dynamics in healthy girls and women. Lower FFA 0 among AAs was independent of insulin 0 and S I. Whether lower FFA 0 is associated with substrate oxidation or risk for obesity remains to be determined. © 2009 The Obesity Society.