The rate of minute ventilation recovery (increase) from 30 to 60 min after subcutaneous administration of morphine at several doses in 20-day-old rats that were opioid naive or made opioid tolerant after previously receiving morphine for 3 days. Recovery rate of ventilation parameters was determined as the least-squares best-fit linear regression of respiratory variables versus time for three measures (30, 45, and 60 min), including weight-normalized minute ventilation (ml·g⁻¹·min⁻²; A) and components tidal volume (V_T; ml·g⁻¹·min⁻¹; B) and respiratory frequency (f; min⁻²; C). Results were only assessed for dose groups where all rats where sedated for all three measures. ⁺Significant differences between morphine-naive versus morphine-tolerant groups at various doses (P < 0.05, between-group ANOVA with the Student’s Newman–Keuls test for post hoc analyses).