In order to elucidate some aspects of the efficacy of orally administered ritodrine, serum concentrations of ritodrine were determined during intravenous and oral therapy. Ritodrine serum concentrations during steady-state intravenous therapy (200 µg/min) ranged from 91.0 to 123.0 ng/ml. Serum concentrations during maintenance therapy with 120 mg ritodrine per os fluctuate between 3.2 and 30.5 ng/ml, depending on the moment of tablet intake. During oral maintenance therapy a striking correlation was found between the maternal heart rate and the serum concentration of ritodrine. The findings suggest that efficacy of orally administered ritodrine is doubtful on pharmacological grounds.