Chronobiologic analyses of data collected during one or more continuous 72-hr spans from 3 heart-transplant patients adhering at diurnal activity (0700-2230) and nocturnal rest (2230-0700) reveal statistically significant circadian rhythmicity in the P-P time interval of recipients (P-P)R and the donor (P-P)D, as well as in the reciprocal measure of the latter, the heart rate. The circadian acrophase for (P-P)D and (P-P)R differ slightly (the 95% confidence arcs overlap). For the group, the acrophases for (P-P)D and heart rate at 0344 (2056 to 0528) and 1656 (0940 to 2100) agree in anti-phase, as expected. For 1 patient studied during 3 separate occasions -1,5 and 12 months post-transplant--the period of the rhythm for the (P-P)R time interval progressively elongated from 23.7 hr (1 month post-transplant) to 26.1 hour (5 months post-transplant) and finally to 32.3 hr (12 months post-transplant and 4 months prior to death) while the (P-P)D remained around 24.0 hr (range 23.8 to 24.6 hr). Change in the rhythm's period in (P-P)R may reflect events associated with rejection, continuing necrosis of cardiac tissue and/or medications, among others.