We measured the heart rate (HR) and respiratory rate (RR) daily during the first week of life in 13 neonates, identified clinically to have LRHR without any identified etiology. They were admitted to the NICU with transient mild unrelated illnesses that resolved without complications. Each daily study was 6 -10 hr. long, during which all R-R intervals and impedance respiration(digitized at 20 Hz) were collected and the data analyzed by a program that computed minute-averaged HR for those periods when respiration was regular and RR was between 30 - 80 bpm. Seven infants had >20% of their averaged HR below 110 bpm within the first 60 hr. of life (LOW group), while the other six had <10% of their averaged HR readings below 110 bpm (control group, CON). During the study period, the CON group had no change in this proportion (daily mean <2%), while the incidence of LRHR for the LOW group decreased from 48% to 7%. HR in the LOW group increased from a mean of 108 on day 1 to 128 on day 7 (mean slope=2.8 +/- 0.6 bpm/day, p <0.01). HR in the CON group (overall mean = 127 bpm) did not change over days (mean slope=0.64 +/- 1.1 bpm/day, ns). Over the 7 days, mean RR remained relatively stable and did not differ between the groups (LOW=46 bpm, CON=48 bpm). While the cause of the LRHR in these infants remains undefined, it can be expected to normalize over the first week.