The use of beta-mimetic tocolytics has been associated with various metabolic derangements which have been the topic of investigational reports. Lactic acidosis has been reported to follow concomitant steroid administration. The purpose of this investigation was to delineate more clearly hyperlactacidemia in patients receiving various tocolytics with and without concomitant use of steroids in a prospective manner. Twenty patients in premature labor who received ritodrine had markedly increased blood levels of lactate (baseline, 1.0 +/- 0.1 mmol/L [SEM] versus 3.5 +/- 0.3 mmol/L [SEM] after 6 hours' intravenous infusion). In the other treatment groups, ritodrine plus hydrocortisone, hydrocortisone alone, and magnesium sulfate alone, lactate levels failed to change significantly. Clinical relevance and implications for metabolic alterations associated with beta-mimetic tocolysis are discussed.