The aim of this study was to evaluate the alarm reaction when hypertensive patients either receiving or not receiving drug therapy have their blood pressure measured (the so-called "White coat effect"). A group of 64 patients (38 males and 26 females, mean age 52 +/- 11 years, OMS stage I-II) was studied. Twenty-three patients were not receiving antihypertensive therapy, 41 patients were regularly taking antihypertensive therapy prescribed by their own doctors. Non-invasive monitoring of arterial pressure (AP) and heart rate (HR) was carried out in all patients. The following parameters were examined in this study: mean of 3 measurements of AP and HR before monitoring (CAP, CHR), daily mean of AP and HR monitoring (MAP, MHR). The following results were obtained: 1) non-treated patients, CAP 157/108 +/- 19/11 mmHg, CHR 83 +/- 11 b/min, MAP 155/100 +/- 20/7 mmHg, MHR 76 +/- 7 b/min; 2) treated patients, CAP 151/96 +/- 21/16 mmHg, CHR 73 +/- 16 b/min, MAP 142/86 +/- 15/12 mmHg, MHR 70 +/- 10 b/min. The two-tailed "t"-test for paired data showed a statistically significant difference (p < 0.001) between diastolic CAP and diastolic MAP in both groups of patients. Systolic CAP was significantly greater than systolic MAP in treated patients (p < 0.01), whereas CHR was significantly higher than MHR in non-treated patients (p < 0.001). These data demonstrate that the alarm reaction to measuring blood pressure is present in both treated and non-treated hypertensive patients. Antihypertensive treatment appears to lessen the chronotropic but not the pressure response to measurement of AP in a hospital setting.