OBJECTIVE To compare the blood pressure (BP) lowering effects of labetalol and nifedipine modified release (MR) in hypertensive pregnant women. We also investigated the effect on the heart rate (HR) and determined the proportion of time spent in target. METHODS This was an exploratory study. Women with chronic hypertension taking either labetalol or nifedipine were offered 24-h ambulatory blood pressure monitoring (ABPM). Sleep, wake and drug ingestion times were self-reported. An indirect response model was used to analyse the systolic BP (SBP), diastolic BP (DBP) and HR time-series; the effect of gestation and type of drug was evaluated. RESULTS Forty-eight women were recruited: 24 in each group. There was no difference in clinical characteristics. In women taking nifedipine there was a positive association between the dose of nifedipine and pre-dose BP p = .002, this was not present in the labetalol group. There was a difference between the drug effects on both the SBP and DBP time-series (p = .014). In comparison to labetalol, there was less variation in day time BP in those women prescribed nifedipine. Women on labetalol spent a larger proportion of time with their DBP below target (<80 mmHg). The HR dynamics were qualitatively different, a stimulatory effect was found with nifedipine compared to an inhibitory effect with labetalol. CONCLUSIONS There are significant and important differences between the BP lowering effects of nifedipine and labetalol. A large randomised control trial is required to investigate the relationship between BP variability and time in target on pregnancy outcomes.