Bone marrow cells were collected from normal dogs, normal dogs made neutropenic with cyclophosphamide, and 11 dogs affected with cyclic hematopoiesis (CH) on 3 consecutive days of separate 12- to 14-day cycles. The mononuclear marrow cells from both groups of control dogs and from the CH dogs on each of 12-cycle days were cultured for 2.5 hr in serum-free media. The amounts of prostaglandins (PGF2 alpha and PGE) and cyclic GMP (cGMP) measured in the media were found to vary with the cycle in the CH dog. PGF2 alpha was highest as the dogs recovered from the neutropenia and lowest 4 days before the onset of the next neutropenic episode. Cyclic GMP was lowest 4-5 days before the onset of neutropenia, then dramatically increased as the neutropenic period approached. Cyclic GMP was highest when PGF2 alpha was lowest. Normal dogs, made neutropenic with a single dose of cyclophosphamide, had elevations of PGF2 alpha but not PGE or cGMP during the recovery period of active granulopoiesis.