To investigate possible circadian and ultradian periodicities for plasma growth hormone in lactating dairy cows, integrated 15-min blood samples taken sequentially over 48 hr from six cows were analyzed by radioimmunoassay. The cows were housed in an environmental chamber at 19 +/- 0.5 degree C, 50% relative humidity, and 16 hr of light and 8 hr of darkness (lights on at 0700 hr); fed daily at 0900 hr; and milked at 0800 and 2000 hr. Peripheral concentrations of growth hormone for all six cows exhibited sinusoidal circadian rhythms with average minima of 4.1 ng/ml at 1820 hr and maxima of 5.3 ng/ml at 0630 hr. Estimated periods of ultradian rhythms for individual cows by spectral analysis, peak identification, and fitting cosine functions using least squares were 71 to 83 min for all cows. No direct relationship between ultradian peaks and milking or feeding was apparent. In conclusion, a circadian rhythm and an ultradian rhythm with a period around 80 min are probably intrinsic to mechanisms regulating peripheral growth hormone concentrations in the lactating dairy cow.