Water potential (Ψ) and osmotic potential (Ψs) were measured weekly through the sequence of seed development in megagametophytes of loblolly pine (Pinus taeda L.). A Wescor 5500XRS vapor pressure osmometer, modified with a cycle hold switch, was used to measure Ψ for whole megagametophytes containing embryos. The Ψ measurements for megagametophytes with embryos removed were also attempted but readings were distorted due to cell lysates from the cut surfaces. Six seasonal sets of megagametophyte Ψ profiles were generated. Megagametophytes from most of the trees examined showed a consistent Ψ pattern: low measurements of -1.0 to -0.75 MPa during early embryo development in late June to early July when embryo Stages 1–2 occur; an increase for one to several weeks to levels of -0.5 to -0.75 MPa, beginning at Stages 3–5 when apical dome formation occurs; followed by a steady drop from -0.85 to -1.7 to -2.0 MPa from Stage 6 onward from late August until just before cone seed release. The Ψs was measured for supernatant from centrifuged frozen-thawed megagametophyte tissue (embryos removed). Megagametophyte Ψs profiles were similar for seeds analyzed from two trees and resembled Ψ observations starting low, rising around Stages 4–7 and then undergoing a major reduction indicating a strong solute accumulation beginning at Stages 7–9.1. Somatic embryos stop growth prematurely in vitro at Stages 8–9.1. The major change in the accumulation of megagametophyte solutes at Stages 8–9.1 correlates with the halt in somatic embryo maturation and suggests that identifying, quantifying and using the major natural soluble compounds that accumulate during mid- to late-stage seed development may be important to improve conifer somatic embryo maturation.