Investigators have reported appreciable practice effects for low frequency pure‐tone absolute thresholds and no practice effects for thresholds at higher frequencies. It was suggested that practicing subjects are learning to discriminate between the signal and a low‐frequency physiologic noise. No practice effect was observed at any frequency when thresholds were measured against a background of random noise. This finding would be predicted by the hypothesis advanced. However, efforts to induce a practice effect for a high frequency tone by introducing a faint, high‐frequency narrow‐band background noise were unsuccessful. Implications of the findings are discussed.