Early developmental stages of the teleost Herichthys cyanoguttatus were reared in tap water or in 0.033% thiourea solution. In the thyroid, small amounts of colloid are secreted from the 2nd day after hatching, with an apparent climax of colloid production by the 5th day. Histologic differences between the thyroid of control and experimental fish are evident on the 4th day after hatching and can be interpreted as the establishment of a pituitary-thyroid feed-back relationship. Very faint aldehyde fuchsin positive (A.F.+) ve cells are present in the pituitary of some fish from the 2nd day after hatching. Differences between the pituitaries of control and experimental fish become evident 2 weeks after hatching: at this time the pituitaries of thiourea-treated fish are enlarged and contain an increased number of A.F.+ ve cells which indicates that these cells include the thyrotrophs. The development of the swim bladder, melanophores, and skin were also affected by thiourea.