Chamomile (Matricaria chamomilla) in the above-ground organs synthesizes and accumulates (Z)- and (E)-2-beta-D: -glucopyranosyloxy-4-methoxy cinnamic acids (GMCA), the precursors of phytoanticipin herniarin (7-methoxycoumarin). The diurnal rhythmicity of the sum of GMCA (maximum before daybreak) and herniarin (acrophase at 10 h 21 min of circadian time) was observed under artificial lighting conditions LD 12:12. The acrophase is the time point of the maximum of the sinusoidal curve fitted to the experimental data. In continuous light, the circadian rhythms of both compounds were first described with similar acrophases of endogenous rhythms; a significantly different result from that in synchronized conditions. The rhythms' mesor (the mean value of the sinusoidal curve fitted to the experimental data) under free-running conditions was not influenced. Abiotic stress under synchronized conditions decreased the average content of GMCA to half of the original level and eliminated the rhythmicity. In contrast, the rhythm of herniarin continued, though its content significantly increased. Nitrogen deficiency resulted in a significant increase in GMCA content, which did not manifest any rhythmicity while the rhythm of herniarin continued. Circadian control of herniarin could be considered as a component of the plant's specialized defence mechanisms.