The testing procedure of Yakubovich's oscillatority property is presented. The procedure is applied for two models of circadian oscillations [J.C. Leloup, A. Goldbeter, A model for circadian rhythms in Drosophila incorporating the formation of a complex between the PER and TIM proteins, J. Biol. Rhythms, 13 (1998) 70-87; J.C. Leloup, D. Gonze, A. Goldbeter, Limit cycle models for circadian rhythms based on transcriptional regulation in Drosophila and Neurospora. J. Biol. Rhythms, 14 (1999) 433-448]. Analytical conditions of these models oscillatority are established and bounds on oscillation amplitude are calculated.