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°C at 0.32°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 · W¹ for ensembles A and B, respectively. Esophageal temperature (T_es), mean weighted skin temperature (_sk), finger temperature (T_fing), and area-weighted heat flux were recorded continuously. Rate of heat debt (S) and integrated mean body temperature (_b,i) were calculated by partitional calorimetry throughout the cold ramp. Extensive peripheral vasoconstriction in the F phase during early periods of the ramp elevated T_es above thermoneutral levels. Shivering thermogenesis (M = M  M_basal, W /m²) was highly correlated with declines in _sk and T_fing (P <0.0001). There was a reduced slope in M as a function of _b,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 _sk and T_es contribute as additive inputs and T_fing has a multiplicative effect on the total control of M during cold transients (R² = 0.9). Endogenous hormonal levels at each menstrual cycle phase, core temperature and _sk inputs, vascular responses, and variations in body heat balance must be considered in quantifying thermoregulatory responses in women during cold stress.