Effects of the menstrual cycle on heat loss and heat production (M) and core and skin temperature responses to cold were studied in six unacclimatized female nonsmokers (18-29 yr of age). Each woman, resting supine, was exposed to a cold transient (ambient temperature = mean radiant temperature = 20 to -5 degrees C at -0.32 degrees C/min, relative humidity = 50 +/- 2%, wind speed = 1 m/s) in the follicular (F) phase (days 2-6) and midluteal (L) phase (days 19-23) of her menstrual cycle. Clothed in each of two ensembles with different thermal resistances, women performed multiple experiments in the F and L phases. Thermal resistance was 0.2 and 0.4 m2 . K . W-1 for ensembles A and B, respectively. Esophageal temperature (Tes), mean weighted skin temperature (Tsk), finger temperature (Tfing), and area-weighted heat flux were recorded continuously. Rate of heat debt (-S) and integrated mean body temperature (Tb,i) were calculated by partitional calorimetry throughout the cold ramp. Extensive peripheral vasoconstriction in the F phase during early periods of the ramp elevated Tes above thermoneutral levels. Shivering thermogenesis (DeltaM = M - Mbasal, W /m2) was highly correlated with declines in Tsk and Tfing (P <0.0001). There was a reduced slope in M as a function of Tb,i in the L phase with ensembles A (P < 0.02) and B (P < 0.01). Heat flux was higher and -S was less in the L phases with ensemble A (P < 0.05). An analytic model revealed that Tsk and Tes contribute as additive inputs and Tfing has a multiplicative effect on the total control of DeltaM during cold transients (R2 = 0.9). Endogenous hormonal levels at each menstrual cycle phase, core temperature and Tsk inputs, vascular responses, and variations in body heat balance must be considered in quantifying thermoregulatory responses in women during cold stress.