Cell density effects were investigated on tumorous hormonal secretion from 10 pituitary adenomas: 3 somatotrophinomas secreting GH and PRL; 7 gonadotrophinomas, 3 co-secreted both FSH and LH, all 7 secreted LH. Enzymatically dispersed tissue was plated out in 24-well plates at 5 ×10<sup>5</sup>, 10<sup>5</sup>, 5 ×10<sup>4</sup> and 10<sup>4</sup> cells/well in serum-free media. Media were collected weekly for 2 weeks. Results: In 3 of 3 somatotrophinomas, GH and PRL secretion was higher (p < 0.05) at both week 1 and 2 from 10<sup>4</sup> cells/well, but similar at other cell densities. In all 3 gonadotrophinomas, the FSH secretory rate was highest at 5 ×10<sup>5</sup> cells/well which fell as cell density decreased. Conversely, in 7 of 7 gonadotrophinomas the LH secretory rate was highest at 10<sup>4</sup> cells/well (p < 0.01) which fell as cell density increased. Conclusion: These data suggest that paracrine factors may modulate tumorous GH, PRL, FSH and LH secretion, and show that FSH and LH secretion vary inversely as cell density increases.