Table 1.

Naloxone-precipitated withdrawal scores in 1, 12, and 23 h heroin access rats.

1 h	4.4 ± 1.9
12 h	34.3 ± 7.7*
23 h	30.8 ± 2.1*

Animals were given a naloxone (1 mg/kg; s.c.) challenge, and the somatic signs of withdrawal were evaluated for 10 min. Withdrawal scores were based on several signs of somatic withdrawal (see Methods).

^*p < 0.01, significantly different from the 1 h group. n = 5–10.