1. Oral micronized progesterone (P) is proposed for the treatment of certain endocrine gynaecological disorders. To examine the effects of P on performance and mood, a randomized, placebo-controlled study of 24 healthy females ages 18-24 years on low-dose oral contraceptives was conducted. 2. Subjects were admitted to the Clinical Research Center on four occasions and received single doses of oral P (300, 600, 1200 mg) or placebo. Blood sampling, psychometric tests and mood scales were administered at baseline and at hourly intervals for 6 h. 3. P doses produced significant dose-related but highly variable increases in plasma P concentrations. Fatigue increased with P doses, although few subjects were objectively drowsy. Very high peak plasma P concentrations, achieved by some subjects at the 1200 mg dose, were associated with decreased information processing and verbal memory function as well as fatigue. 4. We conclude that oral P can safely be prescribed at higher than previously-reported doses, based on evidence of transient behavioural effects only at the highest doses in some subjects who achieved high plasma P concentrations.