Effects of exogenous testosterone on the circadian system of the male domestic canary (Serinus canaria) were studied in light:dark (L:D) cycles with 16 h of bright light (ca. 35 lx) and 8 h of dim light (ca. 3.0 lx) during the nonreproductive (regressive)phase of the annual gonadal cycle. In addition, effects of testosterone implants were tested under L:D cycles (35:3.0 lx) with different L:D ratios and periods (T> 24 h). The question was asked whether male sex hormones affect entrainment properties of the circadian pacemaker system and whether they differentially influence circadian parameters of various behavioral functions (e.g., perch hopping, feeding, vocalization). During the regressive phase, at low levels of endogenous plasma testosterone, application of exogenous testosterone increased locomotor activity and vocalization (songs and calls) to the levels measured previously during the reproductive (progressive)phase. Differential effects of testosterone on amount and "activity time" (α) of perch hopping and feeding were not accompanied by differences in phase relations and entrainment patterns of the two rhythms with the L:D cycle close to the upper limit of the range of entrainment (T> 24 h). It is concluded that exogenous testosterone influences effector processes (locomotor activity, vocalization) but does not directly affect entrainment properties of the circadian pacemaking system in the male canary.