A software package for computer-assisted analysis of antenatal fetal heart rate (FHR) patterns has been commercially available from 1989. Reference ranges for the third trimester of pregnancy have been established. In order to obtain reference values for the second trimester, one hour FHR recordings were made between 20 and 29 weeks gestation in 77 women with uncomplicated pregnancies. There was a significant increase of (medium term) FHR variation and of the number of accelerations from 20 to 29 weeks (r = 0.48, p less than 0.01 and r = 0.34, p less than 0.01 respectively). The lower limit of normal FHR variation (mean--2 SD's) increased from 18 msec at 20 weeks to 28 msec at 28 weeks. Furthermore, a 30 minute recording appeared to be sufficient to obtain a reliable impression of FHR variation. Short term FHR variation, a measure of FHR variation that facilitates the identification of sinusoidal FHR patterns, also increased significantly between 20 and 29 weeks (r = 0.61, p less than 0.01). Delerations (greater than 20 beats/min and greater than or equal to 30 seconds) were present in 54% of the recordings and are a more or less common phenomenon at this age. It is concluded that during the second trimester FHR variation differs considerably from that in the third trimester. This should be taken into account in the interpretation of second trimester FHR records.