Thyroid hormone (TH) action on somatostatin (SRIF) secretion and synthesis by fetal rat brain cells in culture was studied. Cortical and hypothalamic brain cells were maintained as monolayer cultures for 7-10 days. T3, T4, and [3H] phenylalanine [( 3H]Phe) (40 microCi/plate) were added simultaneously for 48 h. Alternately, cultures were pulse labeled with [3H] Phe for only the last 3 h, after being exposed to TH for 45 h. 3H-Labeled SRIF-like material [( 3H]IR-SRIF) was purified by immunoaffinity chromatography and further characterized by gel filtration in Bio-Gel P-10. Total protein synthesis was determined by the incorporation of [3H]Phe into trichloroacetic acid precipitable proteins. Forty eight-hour T3 treatment had a biphasic effect on secretion of IR-SRIF by both cortical and hypothalamic cells. In cortical cells, low doses of T3 (10(-11) M) significantly increased (P less than 0.01) and high T3 doses (10(-7) M) significantly decreased (P less than 0.05) total IR-SRIF (nanograms per plate); control: 2 +/- 0.25; T3 (10(-11) M): 3 +/- 0.3; T3 (10(-7) M): 1.3 +/- 0.1. Similarly, T4 had a significant stimulatory action at 10(-9) M, being inhibitory at 10(-7) M (picograms/plate); control: 290 +/- 20 T4 (10(-9) M): 510 +/- 40; T4 (10(-7) M): 201 +/- 10. When [3H]Phe was added during the 48 h of the experiment, [3H]IR-SRIF synthesis in response to T3 by cortical cells significantly increased after exposure to 10(-11) M (P less than 0.05) and decreased with 10(-7) M (P less than 0.05). When [3H]Phe was added for only the last 3 h or incubation with T3, the action was inhibitory at both 10(-11) M and 10(-7) M. Trichloroacetic acid precipitable material decreased in a dose response manner between T3, 10(-11) M and 10(-7) M. These findings suggest that at this time of brain development, SRIF synthesis by cortical and hypothalamic cells is affected by TH.