The effect of centpiperalone [2-piperazino-4(3H)-quinazolinone monoacetate], a potent hypoglycemic compound on glucose and oxygen uptake, and lactic acid formation by rat hemidiaphragms suspended in glucose-phosphate buffer in Warburg bath at 37 degrees C and 80 shakes/min was studied in vitro. Although centpiperalone enhanced glucose and oxygen uptake by itself, no increment in the above parameters was seen when insulin was added simultaneously along with centpiperalone. Preincubation of hemidiaphragms with centpiperalone and their subsequent incubation with insulin did not indicate any potentiation of insulin action as indicated by uptake of oxygen and glucose. However when 16 times diluted serum from centpiperalone-treated rats was added to hemidiaphragms suspended in glucose-phosphate buffer, significant increase in glycogen content was observed compared to that seen in hemidiaphragms after addition of serum from normal untreated and alloxanized centpiperalone-fed rats. Definite degranulation of B-cells was seen in centpiperalone-fed rats using the aldehyde fuchsin staining technique. These data strongly suggest that centpiperalone induces hypoglycemia through liberation of insulin from B-cells.