In order to assess learning abilities during human sleep, subjects underwent a classical conditioning (tone paired with electric stimulus) using the heart rate as a measure. Subject TY was conditioned in stage 2 sleep while subject AU in slow wave sleep (stage 3 + 4). The conditional responses (CR) formed solely in TY. Two weeks after the preceding study AU received differential reinforcements during each of stage 2 and 3 + 4 at the same time, which was managed by replacing CS+ and CS- between these stages. Conditional discrimination was found in stage 2 only. It is possibly concluded that the difference of conditionability in both sleep stages reflects their depth of sleep.