Certain physiological and behavioral changes were observed in 5 domestic sows. Estrus detd. by a modification of the method used by Young, Dempsey et al. with guinea pigs (stroking back, pressing sides, and moving tail aside, when [female] in heat presents genitalia). The following were observed external signs of heat, spontaneous activity (measured by pedometers harnessed on the animal), vaginal smears (modification of method of Papanicolaou), nasal smears, rectal temp., and potential differences between electrodes located on the flank above the ovaries on either side of the body. "The observation of the external signs of heat, the spontaneous activity and the bioelectrical potential difference allowed the accurate detn. of sexual phases, while the vaginal smear method, the curve of rectal temp. and the nasal smear content did not indicate clear cyclic differences." Records of performances of other animals in a conditioning expt. were analyzed for indications of fluctuations which might correlate with the estrus cycle.