X3 



across a 3C iiy period rats Increased the daily number o^ nicotine infu- 

 sions. In contrast, a stable level of daily infusions was obtained In 

 the present study in 18 (± 1.5 S£M) days. In addition, Hanson et al. 

 (1979) reported that following substitution of saline for nicotine 

 there was a gradual increase (2 days) followed by a gradual decrease 

 (4-6 days) in lever pressing. In the present study a siirilar pattern 

 occurred but the tirne course was within 24 h. The difference in extinc- 

 tion "-ates beti.een the two studies may be a result of the ability of 

 nicotine to exert sturulus control of behav-o-- when in'used over a long 

 (13 s) oeriod of tine (Hanson et al. 1979) vs a relatively short (4 s) 

 infusions interval as used in the present study. This possible explana- 

 tion must be qualified since in the Hanson et al. (1979) study albino 

 rats were studied, they received programrred nicotine infusions ^or sev- 

 """al day? and a between groups design was used. 



The result of delivering noncontingent infusions was a decrease in 

 the nurrber of response contingent infusions. However, the daily nur.ber 

 of infusions (noncontingent + contingent) was stable across the four 

 prograwred interinfusion intervals. These data combined with the sta- 

 ble number of infusions across five ratio values (Fig. 4) suggest that 

 under the present conditions the daily level of nicotine self- 

 administration is at least in part under control of some tissue level 

 of nicotine. 



