The acute effects of partial (focal) epileptic seizures on serum prolactin levels were studied in two groups of patients: (1) 10 with temporal lobe seizures and (2) 11 with seizures that arose from the frontal lobes, recorded on cable video-electroencephalographic telemetry. Six of the eight complex partial seizures of temporal lobe origin were associated with a marked rise in prolactin levels at 10 minutes after onset (rise in levels, from a mean of 279 to 534 mU/L), compared with a rise in only one of the eight frontal lobe complex partial seizures. None of the five simple partial seizures (two of temporal and three of frontal lobe origin) was associated with a marked rise in prolactin levels. This difference in prolactin response following complex partial seizures of frontal and temporal lobe origin may help in the clinical differentiation of these seizures. A failure of prolactin levels to rise does not, however, exclude a diagnosis of complex partial seizures; thus, this measurement will not help in the clinical differentiation of frontal lobe complex partial seizures from psychogenic attacks.