In infants, abnormally high intraocular pressure (IOP) results in excessive eye enlargement. In the study reported here, we investigated whether IOP might be a determinant of ocular elongation using the chick as an animal model. Specifically, we examined IOP changes in (I). normally developing eyes, and (II). eyes undergoing altered growth. In the first case (I). developmental changes in IOP, axial length and refractive error were assessed at approximately daily intervals during early development (days 2-11 post-hatch, n = 8), and at weekly intervals from weeks 1 to 6 (n = 8). In the second case (II). opposite ocular growth responses were elicited using -15 D and +15 D defocusing spectacle lenses fitted monocularly to 8-day-old chicks (n = 8 and 7, respectively). Treated eyes were measured 3 and 7 h after lens application (between 9 and 10 am), as well as 1, 2 and 4 days later, around the same time as the initial lens application. In normal development (I). IOP increased over the first post-hatch week, peaking at 18.0 +/- 5.1 mmHg, and declined slowly thereafter to be back to near 'hatching' values at 6 weeks (12.7 +/- 5.3 mmHg at day 2 cf. 13.4 +/- 1.9 mmHg). Eyes elongated at an approximately linear rate over the entire monitoring period. The -15 D and +15 D lenses (II). produced opposing effects on eye growth, as indicated by axial length changes of + 0.67 +/- 0.25 mm cf. -0.33 +/- 0.17 mm over the 4-day treatment period. Both groups showed decreases in IOP over the first 20 h of lens wear. The faster growing eyes of the -15 D group exhibited a later relative increase in IOP. The described changes in IOP are compatible with, but do not prove, a modulatory influence of IOP on early eye growth.