The effect of porcine vasoactive intestinal polypeptide (VIP) on development of the biphasic insulin release response in cultured fetal rat islets was investigated. Fetal islets, 21.5 days gestational age, were cultured for 7 days in RPMI 1640 culture medium containing either 2.8 or 11.1 mM glucose adn subsequently challenged with 16.7 mM glucose in a perfusion system. Islets were exposed to VIP at a final concentration of 13.2 nM by adding the peptide to the perifusion buffer (acute exposure) or by adding it to the culture medium throughout the culture period (chronic exposure). Islet hormone and DNA contents were also quantitated at the end of the culture period. Acute exposure to VIP resulted in no alterations of the insulin release pattern after culture in the presence of either glucose concentration. However, chronic treatment of islets with 13.2 nM VIP in the presence of 2.8 mM glucose resulted in significant increases in the maximum rate of insulin release during the first phase and the total amount of insulin release during both phases. Similarly, islets cultured in the presence of 11.1 mM glucose and 13.2 nM VIP demonstrated enhanced biphasic insulin release patterns with increased maximum rate and total amount of release during both phases. The presence of VIP and 2.8 mM glucose increased islet glucagon and somatostatin contents, but islet DNA and insulin contents remained unchanged. These findings indicate that VIP plays a significant role in the in vitro development of the biphasic insulin release pattern and may be a factor controlling the maturation of the fetal islet in vivo.