An automatic microcomputer-assisted instrument (Dinamap) for indirect determination of blood pressure was evaluated in 10 healthy women in the last trimester of pregnancy. Blood pressure determined intra-arterially was significantly (p less than 0.001) higher than indirect blood pressure determined with the Dinamap instrument or by manual sphygmomanometry. However, there was no significant difference in the mean change in blood pressure determined by the three techniques under investigation. A close correlation was recorded between systolic and diastolic blood pressure determined intra-arterially and with the Dinamap instrument (r = 0.92, r = 0.90), and between blood pressure determined intra-arterially and indirectly by auscultation (r = 0.93, r = 0.81). There was no significant difference in the reproducability of individual blood pressure values determined intra-arterially (systolic, 2.4 mmHg, diastolic, 3.7 mmHg) or by the Dinamap instrument (systolic, 2.8 mmHg, diastolic, 3.4 mmHg) or by auscultation (systolic, 5.0 mmHg, diastolic, 4.9 mm Hg). The Dinamap instrument proved reliable for the measurement of changes in blood pressure during late pregnancy. The technique eliminates the problem of inter-observer error which otherwise becomes evident when checking blood pressure antenatally.