Testosterone (T) plays a key role in the increase and maintenance of muscle mass and bone density in adult men. Life history theory predicts that environmental stress may prompt a reallocation of such investments to those functions critical to survival. We tested this hypothesis in two studies of rural Bolivian adult men by comparing free T levels and circadian rhythms during late winter, which is especially severe, to those in less arduous seasons. For each pair of salivary TAM/TPM samples (collected in a ∼ 12-h period), circadian rhythm was considered classic (CCLASSIC) if TAM > 110%TPM, reverse (CREVERSE) if TPM > 110%TAM, and flat (CFLAT) otherwise. We tested the hypotheses that mean TAM > mean TPM and that mean TLW < mean TOTHER (LW = late winter, OTHER = other seasons). In Study A, of 115 TPMTAM pairs, 51% = CCLASSIC, 39% = CREVERSE, 10% = CFLAT; in Study B, of 184 TAMTPM pairs, 55% = CCLASSIC, 33% = CREVERSE, 12% = CFLAT. Based on fitting linear mixed models, in both studies TOTHER-AM > TOTHER-PM (A: P = 0.035, B: P = 0.0005) and TOTHER-AM > TLW-AM (A: P = 0.054, B: P = 0.007); TPM did not vary seasonally, and T diurnality was not significant during late winter. T diurnality varied substantially between days within an individual, between individuals and between seasons, but neither T levels nor diurnality varied with age. These patterns may reflect the seasonally varying but unscheduled, life-long, strenuous physical labor that typifies many non-industrialized economies. These results also suggest that single morning samples may substantially underestimate peak circulating T for an individual and, most importantly, that exogenous signals may moderate diurnality and the trajectory of age-related change in the male gonadal axis. Am. J. Hum. Biol., 2009. © 2009 Wiley-Liss, Inc.