Young chickens suppress their regular peeping when they hear a change in the frequency of a tone. Gray and Rubel [J. Acoust. Soc. Am. Suppl. 1 76, S13 (1984)] estimated frequency difference limens in newborn chickens using peep suppressions and a nonstandard adaptive procedure. The definition of a correct response depended on an initial series of control trials used to estimate the distribution of suppressions expected when there was no change in frequency. Consistent improvement in frequency difference limens, approximately 1% in 4 days, was observed. A more controlled procedure involves the presentation of a stimulus and a control trial in pairs. When the duration of suppression is greater on the stimulus than control trial then the response is defined as correct, and transformed up-down rules can be applied as usual. Unfortunately, the data from this procedure are inconsistent. Any increase in frequency selectivity over age is probably greater at high than low frequencies. Difference limens appear to be affected by intensity, but not by the direction of delta F nor temporal characteristics of the stimulus. In conclusion, high variability makes frequency difference limens difficult to estimate in neonates. [Work supported by NIH.]