Pharmacokinetics and ventilatory effects of a single intravenous dose of 0.5 mg/kg of pentazocine were studied in ten children aged 4 to 8 years after ophthalmic surgery. Elimination half-life (mean +/- S.D.) was 3.0 +/- 1.5 hr and clearance 21.8 +/- 5.9 ml/min./kg. The values for Vc, Vss and V beta were 0.73 +/- 0.21, 4.0 +/- 1.2 and 5.3 +/- 2.1 l/kg, respectively. The pharmacokinetic parameters were similar to those of adults. After administration of pentazocine decrease in ventilatory rate and oxygen saturation and increase in end-tidal carbon dioxide were relatively fast and steep. Oxygen saturation of four patients decreased below 90% and in one patient the decrease did not recover instantly and additional oxygen was given for 2 min. No patient needed assisted ventilation. Only clinically insignificant changes in heart rate and mean arterial pressure were observed. The duration of analgesia was 164 +/- 59 min. No serious side-effects appeared.