OBJECTIVE: The aim of the present study was to compare different regimes of intravenous fenoterol tocolysis concerning their side effects. METHODS: A total of 59 patients with intravenous tocolysis due to preterm labor has been studied. They were chronologically alternating and thus randomly stratified into three groups: (a) continuous i.v.-tocolysis and oral application of magnesium sulfate (b) continuous i.v-tocolysis and i.v.-application of magnesium sulfate (c) pulsatile i.v.-application of fenoterol (bolus tocolysis) and oral application of magnesium sulfate. For all groups, heart rate, systolic and diastolic blood pressure, need for beta-blockers, K+, water balance over 24 hours as well as different subjective side effects were quantified. RESULTS: The side effects were found to be significantly less among patients treated with bolus tocolysis and they were also found to be slightly less in continuous tocolysis with i.v.- application of magnesium compared to continuous tocolysis with oral Mg2+ substitution; however, the difference between the two groups treated with continuous tocolysis was not significant. CONCLUSION: The present results suggest that bolus tocolysis shows significantly less side effects compared to the continuous tocolysis and thus it should be favored in clinical practise.