The effects of thyroid hormones (TH) on brain immunoreactive-vasoactive intestinal peptide (IR-VIP) secretion and content in cultured fetal rat cortical cells were studied. Cerebral cortical cells were maintained as monolayer cultures for 14-18 days. T3 or T4 (10–7M) caused a time-dependent decrease in total IR-VIP. Significant suppression was observed following treatment periods of 6 h or longer (24 and 48 h). Depending on the length of time cells had been deprived of TH prior to the addition of exogenous T3 or T4, these two thyroid hormones had different effects on IR-VIP accumulation. Both T3 and T4 caused a dose-dependent suppression or IR-VIP accumulation when there was no deprivation period or when it lasted 4 h. However, a biphasic effect was observed when cells were deprived of TH for 17 and 24 h: low doses of T3 or T4 (from 10–12 to 10–10M) significantly increased (p < 0.05) total IR-VIP, while high T3 or T4 doses (10–8 and 10–7M) caused a significant decrease (p < 0.01). The TH action was furthermore shown to be reversible. After T3 (10–7M) removal and subsequent incubation in serum-free medium for 6, 24 and 48 h, T3-treated and control cells exhibited similar levels of IR-VIP release and content. At this time, a new exposure to T3 (10–7M) again had a suppressive effect. These findings suggest that at this time of brain development, VIP release and content in cerebral cortical cells is affected by TH in a time- and dose-related manner, and furthermore this seems to