Fig. 1. The effects of various subanesthetic doses of halothane on learning, memory, and pain sensitivity in the rat. (  A ) The number of trials required to initially learn the inhibitory avoidance task. Increasing doses above 0.15% necessitated more trials. For comparison with the subsequent retention data, control animals on their first initial step-through had a mean initial crossing latency of 17 ± 9 s. (  B ) Memory retention of the training experience at 24 h. Longer latency to enter the shock compartment represents better memory. A memory enhancement is seen at 0.08%. Significant amnesia is evident only at 0.3%. Data are plotted as median and interquartile range for both learning and memory. (  C ) Pain sensitivity assessed with flinch threshold determination. With this determination, the higher the delivered current tolerated by an animal before its showing a flinch response is, the greater the analgesic effect is. A strong hyperalgesia response (  i.e. , increased pain) is found at 0.08%. Significant analgesia  versus control is evident only at 0.3%. Data are presented as mean and SD. Animal numbers for each group in the learning and memory determination are shown in the graph in  A , whereas animal numbers shown in  C may differ because they are not necessarily the same animals. Some bars are offset from integer values on the x-axis because the bars are placed according to the actual measured agent dose values. *  P < 0.05, **  P < 0.01, ***  P < 0.001 compared with air controls. NS = not significant.